transition to prolog

Cargo.lock

@@ -1,6 +1,6 @@
 [root]
 name = "rusty-wam"
-version = "0.4.0"
+version = "0.5.0"
 dependencies = [
  "lalrpop 0.12.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "lalrpop-util 0.12.5 (registry+https://github.com/rust-lang/crates.io-index)",

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "rusty-wam"
-version = "0.4.0"
+version = "0.5.0"
 authors = ["Mark Thom"]
 
 build = "build.rs"

README.md

@@ -8,9 +8,11 @@ pure Prolog.
 
 ## Progress
 
-The language L3 is implemented as a simple REPL. L3 is pure Prolog --
+Pure Prolog is implemented as a simple REPL. "Pure Prolog" is Prolog
-Prolog without cut, meta- or extra-logical operators, or side effects
+without cut, meta- or extra-logical operators, or side effects of any
-of any kind. No data types apart from atoms are currently supported.
+kind. In terms of the tutorial pacing, the work has progressed to the
+middle of section 5.2. Atoms and lists are the only two data types
+currently supported.
 
 ## Tutorial
 To enter a multi-clause predicate, the brackets ":{" and "}:" are used
@@ -18,11 +20,11 @@ as delimiters. They must be entirely contained with their own lines.
 
 For example,
 ```
-l3> :{
+prolog> :{
 p(f(f(X)), h(W), Y) :- g(W), h(W), f(X).
 p(X, Y, Z) :- h(Y), z(Z).
 }:
-l3> :{
+prolog> :{
 h(x).
 h(y).
 h(z).
@@ -31,28 +33,49 @@ h(z).
 
 Single clause predicates can be entered without brackets, as in
 ```
-l3> p(X) :- q(X).
+prolog> p(X) :- q(X).
-l3> f(s).
+prolog> f(s).
-l3> z(Z).
+prolog> z(Z).
 ```
 
 Queries are issued as
 ```
-l3> ?- p(X, Y, Z).
+prolog> ?- p(X, Y, Z).
 ```
 
 Given the above work, the result of the query will be
 ```
-l3> ?- p(X, Y, Z).
+prolog> ?- p(X, Y, Z).
 yes
 X = _0
 Y = x
 Z = _2
-Press ; to continue or A to abort.
+Press ; to continue or . to abort.
 ```
 
 Pressing ; will backtrack through other possible answers, if any exist.
-Pressing A will abort the search and return to the prompt.
+Pressing . will abort the search and return to the prompt.
+
+Wildcards work as well:
+
+```
+prolog> prolog> :{
+member(X, [X|_]).
+member(X, [_|Xs]) :- member(X, Xs).
+}:
+prolog> ?- member(X, [a, b, c]).
+yes
+X = a
+Press ; to continue or . to abort.
+;
+X = b
+Press ; to continue or . to abort.
+;
+X = c
+Press ; to continue or . to abort.
+;
+no
+```
 
 Note that the values of variables belonging to successful queries are
 printed out, on one line each. Uninstantiated variables are denoted by
@@ -66,8 +89,8 @@ unification on a cyclic term succeeds, and the attempt to write the
 term to a string results in an infinite loop, ie.
 
 ```
-l3> p(W, W).
+prolog> p(W, W).
-l3> ?- p(f(f(W)), W).
+prolog> ?- p(f(f(W)), W).
 yes
 *loops to infinity*
 ```

src/main.rs

@@ -1,13 +1,13 @@
 extern crate termion;
-mod l3;
+mod prolog;
 
-use l3::io::*;
+use prolog::io::*;
-use l3::machine::*;
+use prolog::machine::*;
 
 #[cfg(test)]
 mod tests {
     use super::*;
-    use l3::ast::*;
+    use prolog::ast::*;
     
     fn submit(wam: &mut Machine, buffer: &str) -> EvalResult {
         let result = eval(wam, buffer);
@@ -174,13 +174,59 @@ mod tests {
         assert_eq!(submit(&mut wam, "?- p(X, Y, X).").failed_query(), false);
         assert_eq!(submit(&mut wam, "?- p(f(f(X)), h(f(X)), Y).").failed_query(), true);
     }
+
+    #[test]
+    fn test_queries_on_lists() {
+        let mut wam = Machine::new();
+
+        submit(&mut wam, "p([Z, W]).");
+
+        assert_eq!(submit(&mut wam, "?- p([Z, Z]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- p([Z, W, Y]).").failed_query(), true);
+        assert_eq!(submit(&mut wam, "?- p([Z | W]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- p([Z | [Z]]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- p([Z | [W]]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- p([Z | []]).").failed_query(), true);
+
+        submit(&mut wam, "p([Z, Z]).");
+
+        assert_eq!(submit(&mut wam, "?- p([Z, Z]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- p([Z, W, Y]).").failed_query(), true);
+        assert_eq!(submit(&mut wam, "?- p([Z | W]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- p([Z | [Z]]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- p([Z | [W]]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- p([Z | []]).").failed_query(), true);
+
+        submit(&mut wam, "p([Z]).");
+
+        assert_eq!(submit(&mut wam, "?- p([Z, Z]).").failed_query(), true);
+        assert_eq!(submit(&mut wam, "?- p([Z, W, Y]).").failed_query(), true);
+        assert_eq!(submit(&mut wam, "?- p([Z | W]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- p([Z | [Z]]).").failed_query(), true);
+        assert_eq!(submit(&mut wam, "?- p([Z | [W]]).").failed_query(), true);
+        assert_eq!(submit(&mut wam, "?- p([Z | []]).").failed_query(), false);
+
+        submit(&mut wam, "member(X, [X|Xs]).
+                          member(X, [Y|Xs]) :- member(X, Xs).");
+
+        assert_eq!(submit(&mut wam, "?- member(a, [c, [X, Y]]).").failed_query(), true);
+        assert_eq!(submit(&mut wam, "?- member(c, [a, [X, Y]]).").failed_query(), true);
+        assert_eq!(submit(&mut wam, "?- member(a, [a, [X, Y]]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- member(a, [X, Y, Z]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- member([X, X], [a, [X, Y]]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- member([X, X], [a, [b, c], [b, b], [Z, x], [d, f]]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- member([X, X], [a, [b, c], [b, d], [foo, x], [d, f]]).").failed_query(), true);
+        assert_eq!(submit(&mut wam, "?- member([X, Y], [a, [b, c], [b, b], [Z, x], [d, f]]).").failed_query(), false);
+        assert_eq!(submit(&mut wam, "?- member([X, Y, Y], [a, [b, c], [b, b], [Z, x], [d, f]]).").failed_query(), true);
+        assert_eq!(submit(&mut wam, "?- member([X, Y, Z], [a, [b, c], [b, b], [Z, x], [d, f]]).").failed_query(), true);
+    }
 }
 
-fn l3_repl() {
+fn prolog_repl() {
     let mut wam = Machine::new();
         
     loop {
-        print!("l3> ");
+        print!("prolog> ");
 
         let buffer = read();
 
@@ -199,5 +245,5 @@ fn l3_repl() {
 }
 
 fn main() {
-    l3_repl();
+    prolog_repl();
 }

src/prolog/and_stack.rs

@@ -0,0 +1,80 @@
+use prolog::ast::*;
+
+use std::ops::{Index, IndexMut};
+use std::vec::Vec;
+
+pub struct Frame {
+    pub global_index: usize,
+    pub e: usize,
+    pub cp: CodePtr,
+    perms: Vec<Addr>
+}
+
+impl Frame {
+    fn new(global_index: usize, e: usize, cp: CodePtr, n: usize) -> Self {
+        Frame {
+            global_index: global_index,
+            e: e,
+            cp: cp,
+            perms: vec![Addr::HeapCell(0); n]
+        }
+    }
+}
+
+pub struct AndStack(Vec<Frame>);
+
+impl AndStack {
+    pub fn new() -> Self {
+        AndStack(Vec::new())
+    }
+
+    pub fn push(&mut self, global_index: usize, e: usize, cp: CodePtr, n: usize) {
+        self.0.push(Frame::new(global_index, e, cp, n));
+    }
+
+    pub fn top(&self) -> Option<&Frame> {
+        self.0.last()
+    }
+
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    pub fn clear(&mut self) {
+        self.0.clear()
+    }
+
+    // drop the last n frames.
+    pub fn drop_frames(&mut self, n: usize) {
+        let len = self.0.len();
+        self.0.truncate(len - n);
+    }
+}
+
+impl Index<usize> for AndStack {
+    type Output = Frame;
+
+    fn index(&self, index: usize) -> &Self::Output {
+        self.0.index(index)
+    }
+}
+
+impl IndexMut<usize> for AndStack {
+    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
+        self.0.index_mut(index)
+    }
+}
+
+impl Index<usize> for Frame {
+    type Output = Addr;
+
+    fn index(&self, index: usize) -> &Self::Output {
+        self.perms.index(index - 1)
+    }
+}
+
+impl IndexMut<usize> for Frame {
+    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
+        self.perms.index_mut(index - 1)
+    }
+}

src/prolog/ast.rs

@@ -0,0 +1,342 @@
+use std::cell::Cell;
+use std::collections::HashMap;
+use std::ops::{Add, AddAssign};
+use std::vec::Vec;
+
+pub type Var = String;
+
+pub type Atom = String;
+
+pub enum PredicateClause {
+    Fact(Term),
+    Rule(Rule)
+}
+
+impl PredicateClause {
+    pub fn name(&self) -> &Atom {
+        match self {
+            &PredicateClause::Fact(ref t) => t.name().unwrap(),
+            &PredicateClause::Rule(ref rule) => rule.head.0.name().unwrap()
+        }
+    }
+
+    pub fn arity(&self) -> usize {
+        match self {
+            &PredicateClause::Fact(ref t) => t.arity(),
+            &PredicateClause::Rule(ref rule) => rule.head.0.arity()
+        }
+    }
+}
+
+pub enum TopLevel {
+    Fact(Term),
+    Predicate(Vec<PredicateClause>),
+    Query(Term),
+    Rule(Rule)
+}
+
+#[derive(Clone, Copy)]
+pub enum Level {
+    Deep, Shallow
+}
+
+#[derive(Clone, Copy)]
+pub enum RegType {
+    Perm(usize),
+    Temp(usize)
+}
+
+impl Default for RegType {
+    fn default() -> Self {
+        RegType::Temp(0)
+    }
+}
+
+impl RegType {
+    pub fn reg_num(self) -> usize {
+        match self {
+            RegType::Perm(reg_num) | RegType::Temp(reg_num) => reg_num
+        }
+    }
+
+    pub fn is_perm(self) -> bool {
+        match self {
+            RegType::Perm(_) => true,
+            _ => false
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+pub enum VarReg {
+    ArgAndNorm(RegType, usize),
+    Norm(RegType)
+}
+
+impl VarReg {
+    pub fn norm(self) -> RegType {
+        match self {
+            VarReg::ArgAndNorm(reg, _) | VarReg::Norm(reg) => reg
+        }
+    }
+
+    pub fn root_register(self) -> usize {
+        match self {
+            VarReg::ArgAndNorm(_, root) => root,
+            VarReg::Norm(root) => root.reg_num()
+        }
+    }
+}
+
+impl Default for VarReg {
+    fn default() -> Self {
+        VarReg::Norm(RegType::default())
+    }
+}
+
+#[derive(Clone, PartialEq)]
+pub enum Constant {
+    Atom(Atom),
+    EmptyList
+}
+
+pub enum Term {
+    AnonVar,
+    Clause(Cell<RegType>, Atom, Vec<Box<Term>>),
+    Cons(Cell<RegType>, Box<Term>, Box<Term>),
+    Constant(Cell<RegType>, Constant),
+    Var(Cell<VarReg>, Var)
+}
+
+pub struct Rule {
+    pub head: (Term, Term),
+    pub clauses: Vec<Term>
+}
+
+pub enum TermRef<'a> {
+    AnonVar(Level),
+    Cons(Level, &'a Cell<RegType>, &'a Term, &'a Term),
+    Constant(Level, &'a Cell<RegType>, &'a Constant),
+    Clause(Level, &'a Cell<RegType>, &'a Atom, &'a Vec<Box<Term>>),
+    Var(Level, &'a Cell<VarReg>, &'a Var)
+}
+
+pub enum FactInstruction {
+    GetConstant(Level, Constant, RegType),
+    GetList(Level, RegType),
+    GetStructure(Level, Atom, usize, RegType),
+    GetValue(RegType, usize),
+    GetVariable(RegType, usize),
+    UnifyConstant(Constant),
+    UnifyVariable(RegType),
+    UnifyValue(RegType)
+}
+
+pub enum QueryInstruction {
+    PutConstant(Level, Constant, RegType),
+    PutList(Level, RegType),
+    PutStructure(Level, Atom, usize, RegType),
+    PutValue(RegType, usize),
+    PutVariable(RegType, usize),
+    SetConstant(Constant),
+    SetVariable(RegType),
+    SetValue(RegType)
+}
+
+pub enum ChoiceInstruction {
+    RetryMeElse(usize),
+    TrustMe,
+    TryMeElse(usize)
+}
+
+pub enum ControlInstruction {
+    Allocate(usize),
+    Call(Atom, usize),
+    Deallocate,
+    Proceed
+}
+
+pub type CompiledFact = Vec<FactInstruction>;
+
+pub type CompiledQuery = Vec<QueryInstruction>;
+
+pub enum Line {
+    Choice(ChoiceInstruction),
+    Control(ControlInstruction),
+    Fact(CompiledFact),
+    Query(CompiledQuery)
+}
+
+pub enum LineOrCodeOffset<'a> {
+    Instruction(&'a Line),
+    Offset(usize)
+}
+
+impl<'a> From<&'a Line> for LineOrCodeOffset<'a> {
+    fn from(line: &'a Line) -> Self {
+        LineOrCodeOffset::Instruction(line)
+    }
+}
+
+pub type Code = Vec<Line>;
+
+#[derive(Clone, PartialEq)]
+pub enum Addr {
+    Con(Constant),
+    Lis(usize),
+    HeapCell(usize),
+    StackCell(usize, usize),
+    Str(usize)
+}
+
+impl Addr {
+    pub fn is_ref(&self) -> bool {
+        match self {
+            &Addr::HeapCell(_) | &Addr::StackCell(_, _) => true,
+            _ => false
+        }
+    }
+
+    pub fn as_ref(&self) -> Option<Ref> {
+        match self {
+            &Addr::HeapCell(hc) => Some(Ref::HeapCell(hc)),
+            &Addr::StackCell(fr, sc) => Some(Ref::StackCell(fr, sc)),
+            _ => None
+        }
+    }
+}
+
+impl From<Ref> for Addr {
+    fn from(r: Ref) -> Self {
+        match r {
+            Ref::HeapCell(hc) => Addr::HeapCell(hc),
+            Ref::StackCell(fr, sc) => Addr::StackCell(fr, sc)
+        }
+    }
+}
+
+#[derive(Clone, Copy, PartialEq)]
+pub enum Ref {
+    HeapCell(usize),
+    StackCell(usize, usize)
+}
+
+#[derive(Clone, PartialEq)]
+pub enum HeapCellValue {
+    Con(Constant),
+    Lis(usize),
+    NamedStr(usize, Atom),
+    Ref(Ref),
+    Str(usize)
+}
+
+impl From<Addr> for HeapCellValue {
+    fn from(addr: Addr) -> HeapCellValue {
+        match addr {
+            Addr::Con(constant) =>
+                HeapCellValue::Con(constant),
+            Addr::HeapCell(hc) =>
+                HeapCellValue::Ref(Ref::HeapCell(hc)),
+            Addr::Lis(a) =>
+                HeapCellValue::Lis(a),
+            Addr::StackCell(fr, sc) =>
+                HeapCellValue::Ref(Ref::StackCell(fr, sc)),
+            Addr::Str(hc) =>
+                HeapCellValue::Str(hc)
+        }
+    }
+}
+
+impl HeapCellValue {
+    pub fn as_addr(&self, focus: usize) -> Addr {
+        match self {
+            &HeapCellValue::Con(ref c) => Addr::Con(c.clone()),
+            &HeapCellValue::Lis(a) => Addr::Lis(a),
+            &HeapCellValue::Ref(r) => Addr::from(r),
+            &HeapCellValue::Str(s) => Addr::Str(s),
+            &HeapCellValue::NamedStr(_, _) => Addr::Str(focus)
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+pub enum CodePtr {
+    DirEntry(usize),
+    TopLevel
+}
+
+impl Default for CodePtr {
+    fn default() -> Self {
+        CodePtr::TopLevel
+    }
+}
+
+impl Add<usize> for CodePtr {
+    type Output = CodePtr;
+
+    fn add(self, rhs: usize) -> Self::Output {
+        match self {
+            CodePtr::DirEntry(p) => CodePtr::DirEntry(p + rhs),
+            CodePtr::TopLevel => CodePtr::TopLevel
+        }
+    }
+}
+
+impl AddAssign<usize> for CodePtr {
+    fn add_assign(&mut self, rhs: usize) {
+        match self {
+            &mut CodePtr::DirEntry(ref mut p) => *p += rhs,
+            _ => {}
+        }
+    }
+}
+
+pub type Heap = Vec<HeapCellValue>;
+
+pub type Registers = Vec<Addr>;
+
+impl Term {
+    pub fn subterms(&self) -> usize {
+        match self {
+            &Term::Clause(_, _, ref terms) => terms.len(),
+            _ => 1
+        }
+    }
+
+    pub fn name(&self) -> Option<&Atom> {
+        match self {
+            &Term::Constant(_, Constant::Atom(ref atom))
+            | &Term::Var(_, ref atom)
+            | &Term::Clause(_, ref atom, _) => Some(atom),
+            _ => None
+        }
+    }
+
+    pub fn arity(&self) -> usize {
+        match self {
+            &Term::Clause(_, _, ref child_terms) => child_terms.len(),
+            _ => 0
+        }
+    }
+}
+
+pub type HeapVarDict = HashMap<Var, Addr>;
+
+pub enum EvalResult {
+    EntryFailure,
+    EntrySuccess,
+    InitialQuerySuccess(HeapVarDict),
+    QueryFailure,
+    SubsequentQuerySuccess,
+}
+
+impl EvalResult {
+    #[allow(dead_code)]
+    pub fn failed_query(&self) -> bool {
+        if let &EvalResult::QueryFailure = self {
+            true
+        } else {
+            false
+        }
+    }
+}

src/prolog/codegen.rs

@@ -0,0 +1,546 @@
+use prolog::ast::*;
+use prolog::iterators::{FactIterator, QueryIterator};
+
+use std::cell::Cell;
+use std::cmp::max;
+use std::collections::HashMap;
+use std::vec::Vec;
+
+trait CompilationTarget<'a> {
+    type Iterator : Iterator<Item=TermRef<'a>>;
+
+    fn iter(&'a Term) -> Self::Iterator;
+
+    fn to_structure(Level, Atom, usize, RegType) -> Self;
+    fn to_constant(Level, Constant, RegType) -> Self;
+    fn to_list(Level, RegType) -> Self;
+
+    fn constant_subterm(Constant) -> Self;
+
+    fn argument_to_variable(RegType, usize) -> Self;
+    fn argument_to_value(RegType, usize) -> Self;
+
+    fn subterm_to_variable(RegType) -> Self;
+    fn subterm_to_value(RegType) -> Self;
+
+    fn clause_arg_to_instr(RegType) -> Self;
+}
+
+impl<'a> CompilationTarget<'a> for FactInstruction {
+    type Iterator = FactIterator<'a>;
+
+    fn iter(term: &'a Term) -> Self::Iterator {
+        term.breadth_first_iter()
+    }
+
+    fn to_structure(lvl: Level, atom: Atom, arity: usize, reg: RegType) -> Self {
+        FactInstruction::GetStructure(lvl, atom, arity, reg)
+    }
+
+    fn to_constant(lvl: Level, constant: Constant, reg: RegType) -> Self {
+        FactInstruction::GetConstant(lvl, constant, reg)
+    }
+
+    fn to_list(lvl: Level, reg: RegType) -> Self {
+        FactInstruction::GetList(lvl, reg)
+    }
+
+    fn constant_subterm(constant: Constant) -> Self {
+        FactInstruction::UnifyConstant(constant)
+    }
+
+    fn argument_to_variable(arg: RegType, val: usize) -> Self {
+        FactInstruction::GetVariable(arg, val)
+    }
+
+    fn argument_to_value(arg: RegType, val: usize) -> Self {
+        FactInstruction::GetValue(arg, val)
+    }
+
+    fn subterm_to_variable(val: RegType) -> Self {
+        FactInstruction::UnifyVariable(val)
+    }
+
+    fn subterm_to_value(val: RegType) -> Self {
+        FactInstruction::UnifyValue(val)
+    }
+
+    fn clause_arg_to_instr(val: RegType) -> Self {
+        FactInstruction::UnifyVariable(val)
+    }
+}
+
+impl<'a> CompilationTarget<'a> for QueryInstruction {
+    type Iterator = QueryIterator<'a>;
+
+    fn iter(term: &'a Term) -> Self::Iterator {
+        term.post_order_iter()
+    }
+
+    fn to_structure(lvl: Level, atom: Atom, arity: usize, reg: RegType) -> Self {
+        QueryInstruction::PutStructure(lvl, atom, arity, reg)
+    }
+
+    fn to_constant(lvl: Level, constant: Constant, reg: RegType) -> Self {
+        QueryInstruction::PutConstant(lvl, constant, reg)
+    }
+
+    fn to_list(lvl: Level, reg: RegType) -> Self {
+        QueryInstruction::PutList(lvl, reg)
+    }
+
+    fn constant_subterm(constant: Constant) -> Self {
+        QueryInstruction::SetConstant(constant)
+    }
+
+    fn argument_to_variable(arg: RegType, val: usize) -> Self {
+        QueryInstruction::PutVariable(arg, val)
+    }
+
+    fn argument_to_value(arg: RegType, val: usize) -> Self {
+        QueryInstruction::PutValue(arg, val)
+    }
+
+    fn subterm_to_variable(val: RegType) -> Self {
+        QueryInstruction::SetVariable(val)
+    }
+
+    fn subterm_to_value(val: RegType) -> Self {
+        QueryInstruction::SetValue(val)
+    }
+
+    fn clause_arg_to_instr(val: RegType) -> Self {
+        QueryInstruction::SetValue(val)
+    }
+}
+
+struct TermMarker<'a> {
+    bindings: HashMap<&'a Var, VarReg>,
+    arg_c:    usize,
+    perm_c:   usize,
+    temp_c:   usize
+}
+
+impl<'a> TermMarker<'a> {
+    fn new() -> TermMarker<'a> {
+        TermMarker { bindings: HashMap::new(),
+                     arg_c:    1,
+                     perm_c:   1,
+                     temp_c:   1 }
+    }
+
+    fn reset(&mut self) {
+        self.bindings.clear();
+        self.perm_c = 1;
+    }
+
+    fn contains_var(&self, var: &'a Var) -> bool {
+        self.bindings.contains_key(var)
+    }
+
+    fn get(&self, var: &'a Var) -> VarReg {
+        *self.bindings.get(var).unwrap()
+    }
+
+    fn insert(&mut self, var: &'a Var, r: VarReg) {
+        self.bindings.insert(var, r);
+    }
+    
+    fn mark_non_var(&mut self, lvl: Level, cell: &Cell<RegType>) {
+        let reg_type = cell.get();
+
+        if reg_type.reg_num() == 0 {
+            match lvl {
+                Level::Deep if reg_type.is_perm() => {
+                    let perm = self.perm_c;
+                    self.perm_c += 1;
+                    cell.set(RegType::Perm(perm));
+                },
+                Level::Deep => {
+                    let temp = self.temp_c;
+                    self.temp_c += 1;
+                    cell.set(RegType::Temp(temp));
+                },
+                Level::Shallow if reg_type.is_perm() => {
+                    let arg = self.arg_c;
+                    self.arg_c += 1;
+                    cell.set(RegType::Perm(arg));
+                },
+                Level::Shallow => {
+                    let arg = self.arg_c;
+                    self.arg_c += 1;
+                    cell.set(RegType::Temp(arg));
+                }
+            };
+        }
+    }
+
+    fn mark_old_var(&mut self, lvl: Level, var: &'a Var) -> VarReg
+    {
+        let reg = self.get(var);
+
+        match lvl {
+            Level::Deep => VarReg::Norm(reg.norm()),
+            Level::Shallow => {
+                let reg = VarReg::ArgAndNorm(reg.norm(), self.arg_c);
+
+                self.arg_c += 1;
+                self.insert(var, reg);
+
+                reg
+            }
+        }
+    }
+
+    fn mark_new_var(&mut self, lvl: Level, var: &'a Var, reg: RegType) -> VarReg
+    {
+        let inner_reg = if reg.is_perm() {
+            let perm = self.perm_c;
+            self.perm_c += 1;
+            RegType::Perm(perm)
+        } else {
+            let temp = self.temp_c;
+            self.temp_c += 1;
+            RegType::Temp(temp)
+        };
+
+        let reg = match lvl {
+            Level::Deep => VarReg::Norm(inner_reg),
+            Level::Shallow => {
+                let reg = VarReg::ArgAndNorm(inner_reg, self.arg_c);
+                self.arg_c += 1;
+                reg
+            }
+        };
+        
+        self.insert(var, reg);                
+        reg
+    }
+    
+    fn mark_anon_var(&mut self, lvl: Level) -> VarReg {
+        let inner_reg = {
+            let temp = self.temp_c;
+            self.temp_c += 1;
+            RegType::Temp(temp)
+        };
+        
+        match lvl {
+            Level::Deep => VarReg::Norm(inner_reg),
+            Level::Shallow => {
+                let reg = VarReg::ArgAndNorm(inner_reg, self.arg_c);
+                self.arg_c += 1;
+                reg
+            }
+        }
+    }
+    
+    fn advance_at_head(&mut self, term: &'a Term) {
+        self.arg_c = 1;
+        self.temp_c = max(term.subterms(), self.temp_c) + 1;
+    }
+
+    fn advance(&mut self, term: &'a Term) {
+        self.arg_c = 1;
+        self.temp_c = term.subterms() + 1;
+    }
+}
+
+#[derive(Copy, Clone)]
+enum TermStatus {
+    New, Old, Recurrent
+}
+
+pub struct CodeGenerator<'a> {
+    marker: TermMarker<'a>
+}
+
+type VariableFixture<'a>  = (TermStatus, Vec<&'a Cell<VarReg>>);
+type VariableFixtures<'a> = HashMap<&'a Var, VariableFixture<'a>>;
+
+impl<'a> CodeGenerator<'a> {
+    pub fn new() -> Self {
+        CodeGenerator { marker: TermMarker::new() }
+    }
+
+    pub fn vars(&self) -> &HashMap<&Var, VarReg> {
+        &self.marker.bindings
+    }
+
+    fn to_structure<Target>(&mut self,
+                            lvl: Level,
+                            cell: &'a Cell<RegType>,
+                            name: &'a Atom,
+                            arity: usize)
+                            -> Target
+        where Target: CompilationTarget<'a>
+    {
+        self.marker.mark_non_var(lvl, cell);
+        Target::to_structure(lvl, name.clone(), arity, cell.get())
+    }
+
+    fn to_constant<Target>(&mut self,
+                           lvl: Level,
+                           cell: &'a Cell<RegType>,
+                           constant: &'a Constant)
+                           -> Target
+        where Target: CompilationTarget<'a>
+    {
+        self.marker.mark_non_var(lvl, cell);
+        Target::to_constant(lvl, constant.clone(), cell.get())
+    }
+
+    fn to_list<Target>(&mut self, lvl: Level, cell: &'a Cell<RegType>) -> Target
+        where Target: CompilationTarget<'a>
+    {
+        self.marker.mark_non_var(lvl, cell);
+        Target::to_list(lvl, cell.get())
+    }
+
+    fn constant_subterm<Target>(&mut self,
+                                cell: &'a Cell<RegType>,
+                                constant: &'a Constant)
+                                -> Target
+        where Target: CompilationTarget<'a>
+    {
+        self.marker.mark_non_var(Level::Deep, cell);
+        Target::constant_subterm(constant.clone())
+    }
+    
+    fn anon_var_term<Target>(&mut self, lvl: Level) -> Target
+        where Target: CompilationTarget<'a>
+    {        
+        let reg = self.marker.mark_anon_var(lvl);
+
+        match reg {
+            VarReg::ArgAndNorm(arg, norm) =>
+                Target::argument_to_variable(arg, norm),
+            VarReg::Norm(norm) =>
+                Target::subterm_to_variable(norm)
+        }
+    }
+    
+    fn var_term<Target>(&mut self,
+                        lvl: Level,
+                        cell: &'a Cell<VarReg>,
+                        var: &'a Var)
+                        -> Target
+        where Target: CompilationTarget<'a>
+    {
+        if !self.marker.contains_var(var) {
+            let reg = self.marker.mark_new_var(lvl, var, cell.get().norm());
+            cell.set(reg);
+
+            match reg {
+                VarReg::ArgAndNorm(arg, norm) =>
+                    Target::argument_to_variable(arg, norm),
+                VarReg::Norm(norm) =>
+                    Target::subterm_to_variable(norm)
+            }
+        } else {
+            let reg = self.marker.mark_old_var(lvl, var);
+            cell.set(reg);
+
+            match reg {
+                VarReg::ArgAndNorm(arg, norm) =>
+                    Target::argument_to_value(arg, norm),
+                VarReg::Norm(norm) =>
+                    Target::subterm_to_value(norm)
+            }
+        }
+    }
+
+    fn non_var_subterm<Target>(&mut self, cell: &'a Cell<RegType>) -> Target
+        where Target: CompilationTarget<'a>
+    {
+        self.marker.mark_non_var(Level::Deep, cell);
+        Target::clause_arg_to_instr(cell.get())
+    }
+
+    fn subterm_to_instr<Target>(&mut self, subterm: &'a Term) -> Target
+        where Target: CompilationTarget<'a>
+    {
+        match subterm {
+            &Term::AnonVar =>
+                self.anon_var_term(Level::Deep),
+            &Term::Cons(ref cell, _, _) | &Term::Clause(ref cell, _, _) =>
+                self.non_var_subterm(cell),
+            &Term::Constant(ref cell, ref constant) =>
+                self.constant_subterm(cell, constant),
+            &Term::Var(ref cell, ref var) =>
+                self.var_term(Level::Deep, cell, var)
+        }
+    }
+
+    fn compile_target<Target>(&mut self, term: &'a Term) -> Vec<Target>
+        where Target: CompilationTarget<'a>
+    {
+        let iter       = Target::iter(term);
+        let mut target = Vec::new();
+
+        for term in iter {
+            match term {
+                TermRef::Clause(lvl, cell, atom, terms) => {
+                    target.push(self.to_structure(lvl, cell, atom, terms.len()));
+
+                    for subterm in terms {
+                        target.push(self.subterm_to_instr(subterm.as_ref()));
+                    }
+                },
+                TermRef::Cons(lvl, cell, head, tail) => {
+                    target.push(self.to_list(lvl, cell));
+
+                    target.push(self.subterm_to_instr(head));
+                    target.push(self.subterm_to_instr(tail));
+                },
+                TermRef::Constant(lvl @ Level::Shallow, cell, constant) =>
+                    target.push(self.to_constant(lvl, cell, constant)),
+                TermRef::AnonVar(lvl @ Level::Shallow) =>
+                    target.push(self.anon_var_term(lvl)),
+                TermRef::Var(lvl @ Level::Shallow, ref cell, ref var) =>
+                    target.push(self.var_term(lvl, cell, var)),
+                _ => {}
+            };
+        }
+
+        target
+    }
+
+    fn mark_vars_in_term<Iter>(iter: Iter, vs: &mut VariableFixtures<'a>)
+        where Iter : Iterator<Item=TermRef<'a>>
+    {
+        for term in iter {
+            if let TermRef::Var(_, reg_cell, var) = term {
+                let mut status = vs.entry(var)
+                                   .or_insert((TermStatus::New, Vec::new()));
+
+                status.1.push(reg_cell);
+
+                match status.0 {
+                    TermStatus::Old => status.0 = TermStatus::Recurrent,
+                    _ => {}
+                };
+            }
+        }
+
+        for &mut (ref mut term_status, ref mut cb) in vs.values_mut() {
+            match *term_status {
+                TermStatus::New => *term_status = TermStatus::Old,
+                TermStatus::Recurrent => {
+                    for cell_reg in cb.drain(0..) {
+                        cell_reg.set(VarReg::Norm(RegType::Perm(0)));
+                    }
+                },
+                _ => {}
+            }
+        }
+    }
+
+    fn mark_perm_vars(rule: &'a Rule) -> VariableFixtures {
+        let &Rule { head: (ref p0, ref p1), ref clauses } = rule;
+        let mut vfs = HashMap::new();
+
+        let iter = p0.breadth_first_iter().chain(p1.breadth_first_iter());
+
+        Self::mark_vars_in_term(iter, &mut vfs);
+
+        for term in clauses {
+            Self::mark_vars_in_term(term.breadth_first_iter(), &mut vfs);
+        }
+
+        vfs
+    }
+
+    fn add_conditional_call(compiled_query: &mut Code, term: &Term) {
+        match term {
+            &Term::Constant(_, Constant::Atom(ref atom)) => {
+                let call = ControlInstruction::Call(atom.clone(), 0);
+                compiled_query.push(Line::Control(call));
+            },
+            &Term::Clause(_, ref atom, ref terms) => {
+                let call = ControlInstruction::Call(atom.clone(), terms.len());
+                compiled_query.push(Line::Control(call));
+            },
+            _ => {}
+        }
+    }
+
+    pub fn compile_rule(&mut self, rule: &'a Rule) -> Code {
+        let vfs = Self::mark_perm_vars(&rule);
+        let &Rule { head: (ref p0, ref p1), ref clauses } = rule;
+        let mut perm_vars = 0;
+
+        for &(term_status, _) in vfs.values() {
+            if let TermStatus::Recurrent = term_status {
+                perm_vars += 1;
+            }
+        }
+
+        let mut body = Vec::new();
+
+        body.push(Line::Control(ControlInstruction::Allocate(perm_vars)));
+
+        self.marker.advance(p0);
+        body.push(Line::Fact(self.compile_target(p0)));
+
+        self.marker.advance_at_head(p1);
+        body.push(Line::Query(self.compile_target(p1)));
+
+        Self::add_conditional_call(&mut body, p1);
+
+        body = clauses.iter()
+            .map(|ref term| self.compile_query(term))
+            .fold(body, |mut body, ref mut cqs| {
+                body.append(cqs);
+                body
+            });
+
+        body.push(Line::Control(ControlInstruction::Deallocate));
+        body
+    }
+
+    pub fn compile_fact(&mut self, term: &'a Term) -> Code {
+        self.marker.advance(term);
+
+        let mut compiled_fact = vec![Line::Fact(self.compile_target(term))];
+        let proceed = Line::Control(ControlInstruction::Proceed);
+
+        compiled_fact.push(proceed);
+        compiled_fact
+    }
+
+    pub fn compile_query(&mut self, term: &'a Term) -> Code {
+        self.marker.advance(term);
+
+        let mut compiled_query = vec![Line::Query(self.compile_target(term))];
+        Self::add_conditional_call(&mut compiled_query, term);
+
+        compiled_query
+    }
+
+    pub fn compile_predicate(&mut self, clauses: &'a Vec<PredicateClause>) -> Code
+    {
+        let mut code = Vec::new();
+
+        for (i, clause) in clauses.iter().enumerate() {
+            self.marker.reset();
+
+            let mut clause_code = match clause {
+                &PredicateClause::Fact(ref fact) =>
+                    self.compile_fact(fact),
+                &PredicateClause::Rule(ref rule) =>
+                    self.compile_rule(rule)
+            };
+
+            let choice = match i {
+                0 => ChoiceInstruction::TryMeElse(clause_code.len() + 1),
+                _ if i == clauses.len() - 1 => ChoiceInstruction::TrustMe,
+                _ => ChoiceInstruction::RetryMeElse(clause_code.len() + 1)
+            };
+
+            code.push(Line::Choice(choice));
+            code.append(&mut clause_code);
+        }
+
+        code
+    }
+}

src/prolog/heapview.rs

@@ -0,0 +1,184 @@
+use prolog::and_stack::*;
+use prolog::ast::*;
+
+use std::vec::Vec;
+
+#[derive(Clone, Copy)]
+pub enum TToken {
+    Bar,
+    Comma,
+    LRBracket,
+    LSBracket(usize),
+    Nothing,
+    RRBracket,
+    RSBracket
+}
+
+impl TToken {
+    pub fn as_str(self) -> &'static str {
+        match self {
+            TToken::Bar => " | ",
+            TToken::Comma => ", ",
+            TToken::LRBracket => "(",
+            TToken::LSBracket(_) => "[",
+            TToken::Nothing   => "",
+            TToken::RRBracket => ")",
+            TToken::RSBracket => "]"
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+enum CellRef<'a> {
+    View(CellView<'a>),
+    Redirect(usize),
+    TToken(TToken)
+}
+
+#[derive(Clone, Copy)]
+pub enum CellView<'a> {
+    Con(&'a Constant),
+    HeapVar(usize),
+    StackVar(usize, usize),
+    Str(usize, &'a Atom),
+    TToken(TToken),
+}
+
+pub struct HeapCellViewer<'a> {
+    and_stack: &'a AndStack,
+    heap: &'a Heap,
+    state_stack: Vec<CellRef<'a>>
+}
+
+impl<'a> HeapCellViewer<'a> {
+    fn cell_ref_from_addr(&self, mut focus: &'a Addr) -> CellRef<'a> {
+        loop {
+            match focus {
+                &Addr::Con(ref c) =>
+                    return CellRef::View(CellView::Con(c)),
+                &Addr::Lis(hc) | &Addr::HeapCell(hc) | &Addr::Str(hc) =>
+                    return CellRef::Redirect(hc),
+                &Addr::StackCell(fr, sc) => {
+                    match &self.and_stack[fr][sc] {
+                        &Addr::StackCell(fr1, sc1) => {
+                            if fr1 == fr && sc1 == sc {
+                                return CellRef::View(CellView::StackVar(fr, sc));
+                            }
+                        },
+                        _ => focus = &self.and_stack[fr][sc]
+                    }
+                }
+            }
+        }
+    }
+
+    pub fn new(heap: &'a Heap, and_stack: &'a AndStack, addr: &'a Addr) -> Self
+    {
+        let mut viewer = HeapCellViewer {
+            heap: heap,
+            and_stack: and_stack,
+            state_stack: vec![]
+        };
+
+        let cell_ref = viewer.cell_ref_from_addr(addr);
+        let view = viewer.follow(cell_ref);
+
+        viewer.state_stack.push(CellRef::View(view));
+
+        viewer
+    }
+
+    pub fn remove_token(&mut self, loc: usize) {
+        self.state_stack[loc] = CellRef::TToken(TToken::Nothing);
+    }
+
+    pub fn peek(&mut self) -> Option<CellView<'a>> {
+        let len = self.state_stack.len();
+
+        if len > 0 {
+            let last_elt  = self.state_stack.pop().unwrap();
+            let cell_view = self.follow(last_elt);
+
+            self.state_stack.truncate(len - 1);
+            self.state_stack.push(last_elt);
+
+            Some(cell_view)
+        } else {
+            None
+        }
+    }
+
+    fn from_heap(&mut self, mut focus: usize) -> CellView<'a> {
+        loop {
+            match &self.heap[focus] {
+                &HeapCellValue::Con(ref c) =>
+                    return CellView::Con(c),
+                &HeapCellValue::Lis(a) => {
+                    self.state_stack.push(CellRef::TToken(TToken::RSBracket));
+
+                    self.state_stack.push(CellRef::Redirect(a + 1));
+                    self.state_stack.push(CellRef::TToken(TToken::Bar));
+                    self.state_stack.push(CellRef::Redirect(a));
+
+                    let len = self.state_stack.len() - 4;
+
+                    return CellView::TToken(TToken::LSBracket(len));
+                },
+                &HeapCellValue::NamedStr(arity, ref name) => {
+                    self.state_stack.push(CellRef::TToken(TToken::RRBracket));
+
+                    for i in (2 .. arity + 1).rev() {
+                        self.state_stack.push(CellRef::Redirect(focus + i));
+                        self.state_stack.push(CellRef::TToken(TToken::Comma));
+                    }
+
+                    self.state_stack.push(CellRef::Redirect(focus + 1));
+                    self.state_stack.push(CellRef::TToken(TToken::LRBracket));
+
+                    return CellView::Str(arity, name);
+                },
+                &HeapCellValue::Ref(Ref::HeapCell(hc)) => {
+                    if focus == hc {
+                        return CellView::HeapVar(hc);
+                    } else {
+                        focus = hc;
+                    }
+                },
+                &HeapCellValue::Ref(Ref::StackCell(fr, sc)) => {
+                    match self.cell_ref_from_addr(&self.and_stack[fr][sc]) {
+                        CellRef::View(cell_view) => return cell_view,
+                        CellRef::Redirect(hc)    => focus = hc,
+                        CellRef::TToken(token)   => return CellView::TToken(token)
+                    };
+                },
+                &HeapCellValue::Str(cell_num) =>
+                    focus = cell_num,
+            }
+        }
+    }
+
+    fn follow(&mut self, cell_ref: CellRef<'a>) -> CellView<'a>
+    {
+        match cell_ref {
+            CellRef::Redirect(hc) =>
+                self.from_heap(hc),
+            CellRef::View(cell_view) =>
+                cell_view,
+            CellRef::TToken(term_token) =>
+                CellView::TToken(term_token)
+        }
+    }
+}
+
+
+impl<'a> Iterator for HeapCellViewer<'a> {
+    type Item = CellView<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(cell_ref) = self.state_stack.pop() {
+            Some(self.follow(cell_ref))
+        } else {
+            None
+        }
+    }
+}

src/prolog/io.rs

@@ -0,0 +1,290 @@
+use prolog::ast::*;
+use prolog::codegen::*;
+use prolog::prolog_parser::*;
+use prolog::machine::*;
+
+use termion::raw::IntoRawMode;
+use termion::input::TermRead;
+use termion::event::Key;
+
+use std::io::{Write, stdin, stdout};
+use std::fmt;
+
+impl fmt::Display for Constant {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &Constant::Atom(ref atom) =>
+                write!(f, "{}", atom),        
+            &Constant::EmptyList =>
+                write!(f, "[]")
+        }
+    }
+}
+
+impl fmt::Display for FactInstruction {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &FactInstruction::GetConstant(Level::Shallow, ref constant, ref r) =>
+                write!(f, "get_constant {}, A{}", constant, r.reg_num()),
+            &FactInstruction::GetConstant(Level::Deep, ref constant, ref r) =>
+                write!(f, "get_constant {}, {}", constant, r),
+            &FactInstruction::GetList(Level::Shallow, ref r) =>
+                write!(f, "get_list A{}", r.reg_num()),
+            &FactInstruction::GetList(Level::Deep, ref r) =>
+                write!(f, "get_list {}", r),
+            &FactInstruction::GetStructure(Level::Deep, ref name, ref arity, ref r) =>
+                write!(f, "get_structure {}/{}, {}", name, arity, r),
+            &FactInstruction::GetStructure(Level::Shallow, ref name, ref arity, ref r) =>
+                write!(f, "get_structure {}/{}, A{}", name, arity, r.reg_num()),
+            &FactInstruction::GetValue(ref x, ref a) =>
+                write!(f, "get_value {}, A{}", x, a),
+            &FactInstruction::GetVariable(ref x, ref a) =>
+                write!(f, "get_variable {}, A{}", x, a),
+            &FactInstruction::UnifyConstant(ref constant) =>
+                write!(f, "unify_constant {}", constant),
+            &FactInstruction::UnifyVariable(ref r) =>
+                write!(f, "unify_variable {}", r),
+            &FactInstruction::UnifyValue(ref r) =>
+                write!(f, "unify_value {}", r)
+        }
+    }
+}
+
+impl fmt::Display for QueryInstruction {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &QueryInstruction::PutConstant(Level::Shallow, ref constant, ref r) =>
+                write!(f, "put_constant {}, A{}", constant, r.reg_num()),
+            &QueryInstruction::PutConstant(Level::Deep, ref constant, ref r) =>
+                write!(f, "put_constant {}, {}", constant, r),
+            &QueryInstruction::PutList(Level::Shallow, ref r) =>
+                write!(f, "put_list A{}", r.reg_num()),
+            &QueryInstruction::PutList(Level::Deep, ref r) =>
+                write!(f, "put_list {}", r),
+            &QueryInstruction::PutStructure(Level::Deep, ref name, ref arity, ref r) =>
+                write!(f, "put_structure {}/{}, {}", name, arity, r),
+            &QueryInstruction::PutStructure(Level::Shallow, ref name, ref arity, ref r) =>
+                write!(f, "put_structure {}/{}, A{}", name, arity, r.reg_num()),
+            &QueryInstruction::PutValue(ref x, ref a) =>
+                write!(f, "put_value {}, A{}", x, a),
+            &QueryInstruction::PutVariable(ref x, ref a) =>
+                write!(f, "put_variable {}, A{}", x, a),
+            &QueryInstruction::SetConstant(ref constant) =>
+                write!(f, "set_constant {}", constant),
+            &QueryInstruction::SetVariable(ref r) =>
+                write!(f, "set_variable {}", r),
+            &QueryInstruction::SetValue(ref r) =>
+                write!(f, "set_value {}", r)
+        }
+    }
+}
+
+impl fmt::Display for ControlInstruction {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &ControlInstruction::Allocate(num_cells) =>
+                write!(f, "allocate {}", num_cells),
+            &ControlInstruction::Call(ref name, ref arity) =>
+                write!(f, "call {}/{}", name, arity),
+            &ControlInstruction::Deallocate =>
+                write!(f, "deallocate"),
+            &ControlInstruction::Proceed =>
+                write!(f, "proceed")
+        }
+    }
+}
+
+impl fmt::Display for ChoiceInstruction {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &ChoiceInstruction::TryMeElse(offset) =>
+                write!(f, "try_me_else {}", offset),
+            &ChoiceInstruction::RetryMeElse(offset) =>
+                write!(f, "retry_me_else {}", offset),
+            &ChoiceInstruction::TrustMe =>
+                write!(f, "trust_me")
+        }
+    }
+}
+
+impl fmt::Display for Level {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &Level::Shallow => write!(f, "A"),
+            &Level::Deep => write!(f, "X")
+        }
+    }
+}
+
+impl fmt::Display for VarReg {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &VarReg::Norm(RegType::Perm(reg)) => write!(f, "Y{}", reg),
+            &VarReg::Norm(RegType::Temp(reg)) => write!(f, "X{}", reg),
+            &VarReg::ArgAndNorm(RegType::Perm(reg), arg) =>
+                write!(f, "Y{} A{}", reg, arg),
+            &VarReg::ArgAndNorm(RegType::Temp(reg), arg) =>
+                write!(f, "X{} A{}", reg, arg)
+        }
+    }
+}
+
+impl fmt::Display for RegType {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &RegType::Perm(val) => write!(f, "Y{}", val),
+            &RegType::Temp(val) => write!(f, "X{}", val)
+        }
+    }
+}
+
+
+fn is_consistent(predicate: &Vec<PredicateClause>) -> bool {
+    let name  = predicate.first().unwrap().name();
+    let arity = predicate.first().unwrap().arity();
+
+    for clause in predicate.iter().skip(1) {
+        if !(name == clause.name() && arity == clause.arity()) {
+            return false;
+        }
+    }
+
+    true
+}
+
+#[allow(dead_code)]
+pub fn print_code(code: &Code) {
+    for clause in code {
+        match clause {
+            &Line::Fact(ref fact) =>
+                for fact_instr in fact {
+                    println!("{}", fact_instr);
+                },
+            &Line::Choice(ref choice) =>
+                println!("{}", choice),
+            &Line::Control(ref control) =>
+                println!("{}", control),
+            &Line::Query(ref query) =>
+                for query_instr in query {
+                    println!("{}", query_instr);
+                }
+        }
+    }
+}
+
+pub fn read() -> String {
+    let _ = stdout().flush();
+
+    let mut buffer = String::new();
+    let mut result = String::new();
+
+    let stdin = stdin();
+    stdin.read_line(&mut buffer).unwrap();
+
+    if &*buffer.trim() == ":{" {
+        buffer.clear();
+
+        stdin.read_line(&mut buffer).unwrap();
+
+        while &*buffer.trim() != "}:" {
+            result += buffer.as_str();
+            buffer.clear();
+            stdin.read_line(&mut buffer).unwrap();
+        }
+    } else {
+        result = buffer;
+    }
+
+    result
+}
+
+pub fn eval(wam: &mut Machine, buffer: &str) -> EvalResult
+{
+    let result = parse_TopLevel(buffer);
+    let mut cg = CodeGenerator::new();
+
+    match &result {
+        &Ok(TopLevel::Predicate(ref clauses)) => {
+            if is_consistent(clauses) {
+                let compiled_pred = cg.compile_predicate(clauses);
+                wam.add_predicate(clauses, compiled_pred);                
+
+                EvalResult::EntrySuccess
+            } else {
+                let msg = r"Error: predicate is inconsistent.
+Each predicate must have the same name and arity.";
+
+                println!("{}", msg);
+                EvalResult::EntryFailure
+            }
+        },
+        &Ok(TopLevel::Fact(ref fact)) => {
+            let compiled_fact = cg.compile_fact(&fact);
+            wam.add_fact(fact, compiled_fact);
+            EvalResult::EntrySuccess
+        },
+        &Ok(TopLevel::Rule(ref rule)) => {
+            let compiled_rule = cg.compile_rule(&rule);
+            wam.add_rule(rule, compiled_rule);
+            EvalResult::EntrySuccess
+        },
+        &Ok(TopLevel::Query(ref query)) => {
+            let compiled_query = cg.compile_query(&query);
+            wam.run_query(compiled_query, &cg)            
+        },
+        &Err(_) => {
+            println!("Grammatical error of some kind!");
+            EvalResult::EntryFailure
+        }
+    }
+}
+
+pub fn print(wam: &mut Machine, result: EvalResult) {
+    match result {
+        EvalResult::InitialQuerySuccess(heap_locs) => {
+            println!("yes");
+
+            if heap_locs.is_empty() {
+                return;
+            }
+
+            'outer: loop {
+                let mut result = EvalResult::QueryFailure;
+                let bindings = wam.heap_view(&heap_locs);
+                
+                let stdin  = stdin();
+                let mut stdout = stdout().into_raw_mode().unwrap();
+
+                write!(stdout, "{}\n\r", bindings).unwrap();
+                stdout.flush().unwrap();
+
+                if !wam.or_stack_is_empty() {
+                    write!(stdout, "Press ; to continue or . to abort.\n\r").unwrap();
+                    stdout.flush().unwrap();
+
+                    for c in stdin.keys() {
+                        match c.unwrap() {
+                            Key::Char(';') => {
+                                result = wam.continue_query();
+                                break;
+                            },
+                            Key::Char('.') =>
+                                break 'outer,
+                            _ => {}
+                        }
+                    };
+
+                    if let &EvalResult::QueryFailure = &result {
+                        write!(stdout, "no\n\r").unwrap();
+                        stdout.flush().unwrap();
+                        break;
+                    }
+                } else {
+                    break;
+                }
+            }
+        },
+        EvalResult::QueryFailure => println!("no"),
+        _ => {}
+    };
+}

src/prolog/iterators.rs

@@ -0,0 +1,209 @@
+use prolog::ast::*;
+
+use std::cell::Cell;
+use std::collections::VecDeque;
+use std::vec::Vec;
+
+enum IteratorState<'a> {
+    AnonVar(Level),
+    Clause(Level, usize, &'a Cell<RegType>, &'a Atom, &'a Vec<Box<Term>>),
+    Constant(Level, &'a Cell<RegType>, &'a Constant),
+    InitialCons(Level, &'a Cell<RegType>, &'a Term, &'a Term),
+    FinalCons(Level, &'a Cell<RegType>, &'a Term, &'a Term),
+    RootClause(usize, &'a Vec<Box<Term>>),
+    Var(Level, &'a Cell<VarReg>, &'a Var)
+}
+
+impl<'a> IteratorState<'a> {
+    fn to_state(lvl: Level, term: &'a Term) -> IteratorState<'a> {
+        match term {
+            &Term::AnonVar =>
+                IteratorState::AnonVar(lvl),
+            &Term::Clause(ref cell, ref atom, ref child_terms) =>
+                IteratorState::Clause(lvl, 0, cell, atom, child_terms),
+            &Term::Cons(ref cell, ref head, ref tail) =>
+                IteratorState::InitialCons(lvl, cell, head.as_ref(), tail.as_ref()),
+            &Term::Constant(ref cell, ref constant) =>
+                IteratorState::Constant(lvl, cell, constant),
+            &Term::Var(ref cell, ref var) =>
+                IteratorState::Var(lvl, cell, var)
+        }
+    }
+}
+
+pub struct QueryIterator<'a> {
+    state_stack: Vec<IteratorState<'a>>
+}
+
+impl<'a> QueryIterator<'a> {
+    fn push_clause(&mut self,
+                   lvl: Level,
+                   child_num: usize,
+                   cell: &'a Cell<RegType>,
+                   name: &'a Atom,
+                   child_terms: &'a Vec<Box<Term>>)
+    {
+        self.state_stack.push(IteratorState::Clause(lvl,
+                                                    child_num,
+                                                    cell,
+                                                    name,
+                                                    child_terms));
+    }
+
+    fn push_root_clause(&mut self,
+                        child_num: usize,
+                        child_terms: &'a Vec<Box<Term>>)
+    {
+        self.state_stack.push(IteratorState::RootClause(child_num, child_terms));
+    }
+
+    fn push_subterm(&mut self, lvl: Level, term: &'a Term) {
+        self.state_stack.push(IteratorState::to_state(lvl, term));
+    }
+
+    fn push_final_cons(&mut self,
+                       lvl: Level,
+                       cell: &'a Cell<RegType>,
+                       head: &'a Term,
+                       tail: &'a Term)
+    {
+        self.state_stack.push(IteratorState::FinalCons(lvl, cell, head, tail));
+    }
+
+    fn new(term: &'a Term) -> QueryIterator<'a> {
+        let state = match term {
+            &Term::AnonVar =>
+                IteratorState::AnonVar(Level::Shallow),
+            &Term::Clause(_, _, ref terms) =>
+                IteratorState::RootClause(0, terms),
+            &Term::Cons(ref cell, ref head, ref tail) =>
+                IteratorState::InitialCons(Level::Shallow, cell, head.as_ref(), tail.as_ref()),
+            &Term::Constant(ref cell, ref constant) =>
+                IteratorState::Constant(Level::Shallow, cell, constant),
+            &Term::Var(ref cell, ref var) =>
+                IteratorState::Var(Level::Shallow, cell, var)
+        };
+
+        QueryIterator { state_stack: vec![state] }
+    }
+}
+
+impl<'a> Iterator for QueryIterator<'a> {
+    type Item = TermRef<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        while let Some(iter_state) = self.state_stack.pop() {
+            match iter_state {
+                IteratorState::AnonVar(lvl) =>
+                    return Some(TermRef::AnonVar(lvl)),
+                IteratorState::Clause(lvl, child_num, cell, atom, child_terms) => {
+                    if child_num == child_terms.len() {
+                        return Some(TermRef::Clause(lvl, cell, atom, child_terms));
+                    } else {
+                        self.push_clause(lvl, child_num + 1, cell, atom, child_terms);
+                        self.push_subterm(Level::Deep, child_terms[child_num].as_ref());
+                    }
+                },
+                IteratorState::InitialCons(lvl, cell, head, tail) => {
+                    self.push_final_cons(lvl, cell, head, tail);
+                    self.push_subterm(Level::Deep, head);
+                    self.push_subterm(Level::Deep, tail);
+                },
+                IteratorState::FinalCons(lvl, cell, head, tail) =>
+                    return Some(TermRef::Cons(lvl, cell, head, tail)),
+                IteratorState::Constant(lvl, cell, constant) =>
+                    return Some(TermRef::Constant(lvl, cell, constant)),
+                IteratorState::RootClause(child_num, child_terms) => {
+                    if child_num == child_terms.len() {
+                        return None;
+                    } else {
+                        self.push_root_clause(child_num + 1, child_terms);
+                        self.push_subterm(Level::Shallow, child_terms[child_num].as_ref());
+                    }
+                },
+                IteratorState::Var(lvl, cell, var) =>
+                    return Some(TermRef::Var(lvl, cell, var))
+            };
+        }
+
+        None
+    }
+}
+
+pub struct FactIterator<'a> {
+    state_queue: VecDeque<IteratorState<'a>>,
+}
+
+impl<'a> FactIterator<'a> {
+    fn push_subterm(&mut self, lvl: Level, term: &'a Term) {
+        self.state_queue.push_back(IteratorState::to_state(lvl, term));
+    }
+
+    fn new(term: &'a Term) -> FactIterator<'a> {
+        let states = match term {
+            &Term::AnonVar =>
+                vec![IteratorState::AnonVar(Level::Shallow)],
+            &Term::Clause(_, _, ref terms) =>
+                vec![IteratorState::RootClause(0, terms)],
+            &Term::Cons(ref cell, ref head, ref tail) =>
+                vec![IteratorState::InitialCons(Level::Shallow,
+                                                cell,
+                                                head.as_ref(),
+                                                tail.as_ref())],
+            &Term::Constant(ref cell, ref constant) =>
+                vec![IteratorState::Constant(Level::Shallow, cell, constant)],
+            &Term::Var(ref cell, ref var) =>
+                vec![IteratorState::Var(Level::Shallow, cell, var)]
+        };
+
+        FactIterator { state_queue: VecDeque::from(states) }
+    }
+}
+
+impl<'a> Iterator for FactIterator<'a> {
+    type Item = TermRef<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        while let Some(state) = self.state_queue.pop_front() {
+            match state {
+                IteratorState::AnonVar(lvl) =>
+                    return Some(TermRef::AnonVar(lvl)),
+                IteratorState::Clause(lvl, _, cell, atom, child_terms) => {
+                    for child_term in child_terms {
+                        self.push_subterm(Level::Deep, child_term);
+                    }
+
+                    return Some(TermRef::Clause(lvl, cell, atom, child_terms));
+                },
+                IteratorState::InitialCons(lvl, cell, head, tail) => {
+                    self.push_subterm(Level::Deep, head);
+                    self.push_subterm(Level::Deep, tail);
+
+                    return Some(TermRef::Cons(lvl, cell, head, tail));
+                },
+                IteratorState::Constant(lvl, cell, constant) =>
+                    return Some(TermRef::Constant(lvl, cell, constant)),
+                IteratorState::RootClause(_, child_terms) => {
+                    for child_term in child_terms {
+                        self.push_subterm(Level::Shallow, child_term);
+                    }
+                },
+                IteratorState::Var(lvl, cell, var) =>
+                    return Some(TermRef::Var(lvl, cell, var)),
+                _ => {}
+            }
+        }
+
+        None
+    }
+}
+
+impl Term {
+    pub fn post_order_iter(&self) -> QueryIterator {
+        QueryIterator::new(self)
+    }
+
+    pub fn breadth_first_iter(&self) -> FactIterator {
+        FactIterator::new(self)
+    }
+}

src/prolog/machine.rs

@@ -0,0 +1,784 @@
+use prolog::ast::*;
+use prolog::codegen::*;
+use prolog::heapview::*;
+use prolog::and_stack::*;
+use prolog::or_stack::*;
+
+use std::collections::HashMap;
+use std::ops::{Index, IndexMut};
+use std::vec::Vec;
+
+#[derive(Clone, Copy)]
+enum MachineMode {
+    Read,
+    Write
+}
+
+struct MachineState {
+    h: usize,
+    s: usize,
+    p: CodePtr,
+    b: usize,
+    e: usize,
+    num_of_args: usize,
+    cp: CodePtr,
+    fail: bool,
+    heap: Heap,
+    mode: MachineMode,
+    and_stack: AndStack,
+    or_stack: OrStack,
+    registers: Registers,
+    trail: Vec<Ref>,
+    tr: usize,
+    hb: usize
+}
+
+type CodeDir = HashMap<(Atom, usize), usize>;
+
+pub struct Machine {
+    ms: MachineState,
+    code: Code,
+    code_dir: CodeDir
+}
+
+impl Index<RegType> for MachineState {
+    type Output = Addr;
+
+    fn index(&self, reg: RegType) -> &Self::Output {
+        match reg {
+            RegType::Temp(temp) => &self.registers[temp],
+            RegType::Perm(perm) => {
+                let e = self.e;
+                &self.and_stack[e][perm]
+            }
+        }
+    }
+}
+
+impl IndexMut<RegType> for MachineState {
+    fn index_mut(&mut self, reg: RegType) -> &mut Self::Output {
+        match reg {
+            RegType::Temp(temp) => &mut self.registers[temp],
+            RegType::Perm(perm) => {
+                let e = self.e;
+                &mut self.and_stack[e][perm]
+            }
+        }
+    }
+}
+
+impl Machine {
+    pub fn new() -> Self {
+        Machine {
+            ms: MachineState::new(),
+            code: Vec::new(),
+            code_dir: HashMap::new()
+        }
+    }
+
+    pub fn failed(&self) -> bool {
+        self.ms.fail
+    }
+
+    pub fn add_fact(&mut self, fact: &Term, mut code: Code) {
+        if fact.name().is_some() {
+            let p = self.code.len();
+            let name  = fact.name().unwrap().clone();
+            let arity = fact.arity();
+
+            self.code.append(&mut code);
+            self.code_dir.insert((name, arity), p);
+        }
+    }
+
+    pub fn add_rule(&mut self, rule: &Rule, mut code: Code) {
+        if rule.head.0.name().is_some() {
+            let p = self.code.len();
+            let name  = rule.head.0.name().unwrap().clone();
+            let arity = rule.head.0.arity();
+
+            self.code.append(&mut code);
+            self.code_dir.insert((name, arity), p);
+        }
+    }
+
+    pub fn add_predicate(&mut self, pred: &Vec<PredicateClause>, mut code: Code)
+    {
+        let p = self.code.len();
+        let name  = pred.first().unwrap().name().clone();
+        let arity = pred.first().unwrap().arity();
+
+        self.code.append(&mut code);
+        self.code_dir.insert((name, arity), p);
+    }
+
+    fn execute_instr<'a>(&mut self, instr_src: LineOrCodeOffset<'a>) -> bool
+    {
+        let mut instr = match instr_src {
+            LineOrCodeOffset::Instruction(instr) => instr,
+            LineOrCodeOffset::Offset(p) => &self.code[p]
+        };
+
+        loop {
+            match instr {
+                &Line::Choice(ref choice_instr) =>
+                    self.ms.execute_choice_instr(choice_instr),
+                &Line::Fact(ref fact) => {
+                    for fact_instr in fact {
+                        if self.failed() {
+                            break;
+                        }
+
+                        self.ms.execute_fact_instr(&fact_instr);
+                    }
+                    self.ms.p += 1;
+                },
+                &Line::Query(ref query) => {
+                    for query_instr in query {
+                        if self.failed() {
+                            break;
+                        }
+
+                        self.ms.execute_query_instr(&query_instr);
+                    }
+                    self.ms.p += 1;
+                },
+                &Line::Control(ref control_instr) =>
+                    self.ms.execute_ctrl_instr(&self.code_dir, control_instr),
+            }
+
+            if self.failed() {
+                let p = self.ms
+                            .or_stack
+                            .top()
+                            .map(|fr| fr.bp)
+                            .unwrap_or_default();
+
+                if let CodePtr::TopLevel = p {
+                    return false;
+                } else {
+                    self.ms.fail = false;
+                    self.ms.p = p;
+                }
+            }
+
+            match self.ms.p {
+                CodePtr::DirEntry(p) if p < self.code.len() =>
+                    instr = &self.code[p],
+                _ => break
+            }
+        }
+
+        true
+    }
+
+    pub fn heap_view(&self, var_dir: &HeapVarDict) -> String {
+        let mut result = String::new();
+
+        for (var, addr) in var_dir {
+            let mut viewer = HeapCellViewer::new(&self.ms.heap,
+                                                 &self.ms.and_stack,
+                                                 addr);
+
+            if result != "" {
+                result += "\n\r";
+            }
+
+            result += var.as_str();
+            result += " = ";
+
+            while let Some(view) = viewer.next() {
+                match view {
+                    CellView::Con(&Constant::EmptyList) =>
+                        result += "[]",
+                    CellView::Con(&Constant::Atom(ref atom)) =>
+                        result += atom.as_str(),
+                    CellView::HeapVar(cell_num) => {
+                        result += "_";
+                        result += cell_num.to_string().as_str();
+                    },
+                    CellView::StackVar(fr, sc) => {
+                        result += "_";
+                        result += fr.to_string().as_str();
+                        result += "_";
+                        result += sc.to_string().as_str();
+                    },
+                    CellView::Str(_, ref name) =>
+                        result += name.as_str(),
+                    CellView::TToken(TToken::Bar) => {
+                        match viewer.peek() {
+                            Some(CellView::Con(&Constant::EmptyList)) => {
+                                viewer.next();
+                            },
+                            Some(CellView::TToken(TToken::LSBracket(loc))) => {
+                                result += ", ";
+
+                                viewer.next();
+                                viewer.remove_token(loc);
+                            },
+                            _ => result += " | "
+                        };
+                    },
+                    CellView::TToken(token) =>
+                        result += token.as_str()
+                };
+            }
+        }
+
+        result
+    }
+
+    pub fn run_query(&mut self, code: Code, cg: &CodeGenerator) -> EvalResult
+    {
+        let mut succeeded = true;
+        let mut heap_locs = HashMap::new();
+
+        for instr in code.iter().take(1) {
+            succeeded = self.execute_instr(LineOrCodeOffset::from(instr));
+        }
+
+        if succeeded {
+            for (var, vr) in cg.vars() {
+                let addr = self.ms.registers[vr.root_register()].clone();
+                heap_locs.insert((*var).clone(), addr);
+            }
+
+            for instr in code.iter().skip(1) {
+                succeeded = self.execute_instr(LineOrCodeOffset::from(instr));
+                if !succeeded {
+                    break;
+                }
+            }
+        }
+
+        if succeeded {
+            EvalResult::InitialQuerySuccess(heap_locs)
+        } else {
+            EvalResult::QueryFailure
+        }
+    }
+
+    pub fn or_stack_is_empty(&self) -> bool {
+        self.ms.or_stack.is_empty()
+    }
+
+    pub fn continue_query(&mut self) -> EvalResult
+    {
+        if !self.or_stack_is_empty() {
+            let b = self.ms.b;
+            self.ms.p = self.ms.or_stack[b].bp;
+
+            let succeeded = if let CodePtr::DirEntry(p) = self.ms.p {
+                self.execute_instr(LineOrCodeOffset::Offset(p))
+            } else {
+                false
+            };
+
+            if succeeded {
+                EvalResult::SubsequentQuerySuccess
+            } else {
+                EvalResult::QueryFailure
+            }
+        } else {
+            EvalResult::QueryFailure
+        }
+    }
+
+    pub fn reset(&mut self) {
+        self.ms.reset();
+    }
+}
+
+impl MachineState {
+    fn new() -> MachineState {
+        MachineState { h: 0,
+                       s: 0,
+                       p: CodePtr::TopLevel,
+                       b: 0,
+                       e: 0,
+                       num_of_args: 0,
+                       cp: CodePtr::TopLevel,
+                       fail: false,
+                       heap: Vec::with_capacity(256),
+                       mode: MachineMode::Write,
+                       and_stack: AndStack::new(),
+                       or_stack: OrStack::new(),
+                       registers: vec![Addr::HeapCell(0); 64],
+                       trail: Vec::new(),
+                       tr: 0,
+                       hb: 0
+        }
+    }
+
+    fn num_frames(&self) -> usize {
+        self.and_stack.len() + self.or_stack.len()
+    }
+
+    fn store(&self, a: Addr) -> Addr {
+        match a {
+            Addr::HeapCell(r)       => self.heap[r].as_addr(r),
+            Addr::StackCell(fr, sc) => self.and_stack[fr][sc].clone(),
+            addr                    => addr
+        }
+    }
+
+    fn deref(&self, a: Addr) -> Addr {
+        let mut a = a;
+
+        loop {
+            let value = self.store(a.clone());
+
+            if value.is_ref() && value != a {
+                a = value;
+                continue;
+            }
+
+            return a;
+        };
+    }
+
+    fn bind(&mut self, r1: Ref, a2: Addr) {
+        let t2 = self.store(a2);
+
+        match r1 {
+            Ref::StackCell(fr, sc) =>
+                self.and_stack[fr][sc] = t2,
+            Ref::HeapCell(hc) =>
+                self.heap[hc] = HeapCellValue::from(t2)
+        };
+
+        self.trail(r1);
+    }
+
+    fn unify(&mut self, a1: Addr, a2: Addr) {
+        let mut pdl = vec![a1, a2];
+
+        self.fail = false;
+
+        while !(pdl.is_empty() || self.fail) {
+            let d1 = self.deref(pdl.pop().unwrap());
+            let d2 = self.deref(pdl.pop().unwrap());
+
+            if d1 != d2 {
+                match (self.store(d1.clone()), self.store(d2.clone())) {
+                    (Addr::HeapCell(hc), _) =>
+                        self.bind(Ref::HeapCell(hc), d2),
+                    (_, Addr::HeapCell(hc)) =>
+                        self.bind(Ref::HeapCell(hc), d1),
+                    (Addr::StackCell(fr, sc), _) =>
+                        self.bind(Ref::StackCell(fr, sc), d2),
+                    (_, Addr::StackCell(fr, sc)) =>
+                        self.bind(Ref::StackCell(fr, sc), d1),
+                    (Addr::Lis(a1), Addr::Lis(a2)) => {
+                        pdl.push(Addr::HeapCell(a1));
+                        pdl.push(Addr::HeapCell(a2));
+
+                        pdl.push(Addr::HeapCell(a1 + 1));
+                        pdl.push(Addr::HeapCell(a2 + 1));
+                    },
+                    (Addr::Con(c1), Addr::Con(c2)) => {
+                        if c1 != c2 {
+                            self.fail = true;
+                        }
+                    },
+                    (Addr::Str(a1), Addr::Str(a2)) => {
+                        let r1 = &self.heap[a1];
+                        let r2 = &self.heap[a2];
+
+                        if let &HeapCellValue::NamedStr(n1, ref f1) = r1 {
+                            if let &HeapCellValue::NamedStr(n2, ref f2) = r2 {
+                                if n1 == n2 && *f1 == *f2 {
+                                    for i in 1 .. n1 + 1 {
+                                        pdl.push(Addr::HeapCell(a1 + i));
+                                        pdl.push(Addr::HeapCell(a2 + i));
+                                    }
+
+                                    continue;
+                                }
+                            }
+                        }
+
+                        self.fail = true;
+                    },
+                    _ => self.fail = true
+                };
+            }
+        }
+    }
+
+    fn trail(&mut self, r: Ref) {
+        match r {
+            Ref::HeapCell(hc) => {
+                if hc < self.hb {
+                    self.trail.push(r);
+                    self.tr += 1;
+                }
+            },
+            Ref::StackCell(fr, _) => {
+                let fr_gi = self.and_stack[fr].global_index;
+                let b_gi  = if !self.or_stack.is_empty() {
+                    self.or_stack[self.b].global_index
+                } else {
+                    0
+                };
+
+                if fr_gi < b_gi {
+                    self.trail.push(r);
+                    self.tr += 1;
+                }
+            }
+        }
+    }
+
+    fn unwind_trail(&mut self, a1: usize, a2: usize) {
+        for i in a1 .. a2 {
+            match self.trail[i] {
+                Ref::HeapCell(r) =>
+                    self.heap[r] = HeapCellValue::Ref(Ref::HeapCell(r)),
+                Ref::StackCell(fr, sc) =>
+                    self.and_stack[fr][sc] = Addr::StackCell(fr, sc)
+            }
+        }
+    }
+
+    fn execute_fact_instr(&mut self, instr: &FactInstruction) {
+        match instr {
+            &FactInstruction::GetConstant(_, ref constant, reg) => {
+                let addr = self.deref(self[reg].clone());
+
+                match self.store(addr) {
+                    Addr::HeapCell(hc) => {
+                        self.heap[hc] = HeapCellValue::Con(constant.clone());
+                        self.trail(Ref::HeapCell(hc));
+                    },
+                    Addr::StackCell(fr, sc) => {
+                        self.and_stack[fr][sc] = Addr::Con(constant.clone());
+                        self.trail(Ref::StackCell(fr, sc));
+                    },
+                    Addr::Con(c) => {
+                        if c != *constant {
+                            self.fail = true;
+                        }
+                    },
+                    _ => self.fail = true
+                };
+            },
+            &FactInstruction::GetList(_, reg) => {
+                let addr = self.deref(self[reg].clone());
+
+                match self.store(addr.clone()) {
+                    Addr::HeapCell(hc) => {
+                        let h = self.h;
+
+                        self.heap.push(HeapCellValue::Lis(h+1));
+                        self.bind(Ref::HeapCell(hc), Addr::HeapCell(h));
+
+                        self.h += 1;
+                        self.mode = MachineMode::Write;
+                    },
+                    Addr::StackCell(fr, sc) => {
+                        let h = self.h;
+
+                        self.heap.push(HeapCellValue::Lis(h+1));
+                        self.bind(Ref::StackCell(fr, sc), Addr::HeapCell(h));
+
+                        self.h += 1;
+                        self.mode = MachineMode::Write;
+                    },
+                    Addr::Lis(a) => {
+                        self.s = a;
+                        self.mode = MachineMode::Read;
+                    },
+                    _ => self.fail = true
+                };
+            },
+            &FactInstruction::GetStructure(_, ref name, arity, reg) => {
+                let addr = self.deref(self[reg].clone());
+
+                match self.store(addr.clone()) {
+                    Addr::Str(a) => {
+                        let result = &self.heap[a];
+
+                        if let &HeapCellValue::NamedStr(narity, ref str) = result {
+                            if narity == arity && *name == *str {
+                                self.s = a + 1;
+                                self.mode = MachineMode::Read;
+                            } else {
+                                self.fail = true;
+                            }
+                        }
+                    },
+                    Addr::HeapCell(_) | Addr::StackCell(_, _) => {
+                        self.heap.push(HeapCellValue::Str(self.h + 1));
+                        self.heap.push(HeapCellValue::NamedStr(arity, name.clone()));
+
+                        let h = self.h;
+
+                        self.bind(addr.as_ref().unwrap(), Addr::HeapCell(h));
+
+                        self.h += 2;
+                        self.mode = MachineMode::Write;
+                    },
+                    _ => self.fail = true
+                };
+            },
+            &FactInstruction::GetVariable(norm, arg) =>
+                self[norm] = self.registers[arg].clone(),
+            &FactInstruction::GetValue(norm, arg) => {
+                let norm_addr = self[norm].clone();
+                let reg_addr  = self.registers[arg].clone();
+
+                self.unify(norm_addr, reg_addr);
+            },
+            &FactInstruction::UnifyConstant(ref c) => {
+                match self.mode {
+                    MachineMode::Read  => {
+                        let addr = self.deref(Addr::HeapCell(self.s));
+
+                        match self.store(addr) {
+                            Addr::HeapCell(hc) =>
+                                self.heap[hc] = HeapCellValue::Con(c.clone()),
+                            Addr::StackCell(fr, sc) =>
+                                self.and_stack[fr][sc] = Addr::Con(c.clone()),
+                            Addr::Con(c1) => {
+                                if c1 != *c {
+                                    self.fail = true;
+                                }
+                            },
+                            _ => self.fail = true
+                        };
+                    },
+                    MachineMode::Write => {
+                        self.heap.push(HeapCellValue::Con(c.clone()));
+                        self.h += 1;
+                    }
+                };
+            },
+            &FactInstruction::UnifyVariable(reg) => {
+                match self.mode {
+                    MachineMode::Read  =>
+                        self[reg] = self.heap[self.s].as_addr(self.s),
+                    MachineMode::Write => {
+                        let h = self.h;
+
+                        self.heap.push(HeapCellValue::Ref(Ref::HeapCell(h)));
+                        self[reg] = Addr::HeapCell(self.h);
+                        self.h += 1;
+                    }
+                };
+
+                self.s += 1;
+            },
+            &FactInstruction::UnifyValue(reg) => {
+                let s = self.s;
+
+                match self.mode {
+                    MachineMode::Read  => {
+                        let reg_addr = self[reg].clone();
+                        self.unify(reg_addr, Addr::HeapCell(s));
+                    },
+                    MachineMode::Write => {
+                        let heap_val = self.store(self[reg].clone());
+                        self.heap.push(HeapCellValue::from(heap_val));
+                        self.h += 1;
+                    }
+                };
+
+                self.s += 1;
+            }
+        };
+    }
+
+    fn execute_query_instr(&mut self, instr: &QueryInstruction) {
+        match instr {
+            &QueryInstruction::PutConstant(_, ref constant, reg) =>
+                self[reg] = Addr::Con(constant.clone()),
+            &QueryInstruction::PutList(_, reg) =>
+                self[reg] = Addr::Lis(self.h),
+            &QueryInstruction::PutStructure(_, ref name, arity, reg) => {
+                self.heap.push(HeapCellValue::NamedStr(arity, name.clone()));
+                self[reg] = Addr::Str(self.h);
+                self.h += 1;
+            },
+            &QueryInstruction::PutValue(norm, arg) =>
+                self.registers[arg] = self[norm].clone(),
+            &QueryInstruction::PutVariable(norm, arg) => {
+                let h = self.h;
+                self.heap.push(HeapCellValue::Ref(Ref::HeapCell(h)));
+
+                self[norm] = Addr::HeapCell(h);
+                self.registers[arg] = Addr::HeapCell(h);
+
+                self.h += 1;
+            },
+            &QueryInstruction::SetConstant(ref constant) => {
+                self.heap.push(HeapCellValue::Con(constant.clone()));
+                self.h += 1;
+            },
+            &QueryInstruction::SetVariable(reg) => {
+                let h = self.h;
+                self.heap.push(HeapCellValue::Ref(Ref::HeapCell(h)));
+                self[reg] = Addr::HeapCell(h);
+
+                self.h += 1;
+            },
+            &QueryInstruction::SetValue(reg) => {
+                let heap_val = self[reg].clone();
+                self.heap.push(HeapCellValue::from(heap_val));
+
+                self.h += 1;
+            },
+        }
+    }
+
+    fn execute_ctrl_instr(&mut self, code_dir: &CodeDir, instr: &ControlInstruction)
+    {
+        match instr {
+            &ControlInstruction::Allocate(num_cells) => {
+                let num_frames = self.num_frames();
+
+                self.and_stack.push(num_frames + 1, self.e, self.cp, num_cells);
+
+                self.e = self.and_stack.len() - 1;
+                self.p += 1;
+            },
+            &ControlInstruction::Call(ref name, arity) => {
+                let compiled_tl_index = code_dir.get(&(name.clone(), arity))
+                                                .map(|index| *index);
+
+                match compiled_tl_index {
+                    Some(compiled_tl_index) => {
+                        self.cp = self.p + 1;
+                        self.num_of_args = arity;
+                        self.p  = CodePtr::DirEntry(compiled_tl_index);
+                    },
+                    None => self.fail = true
+                };
+            },
+            &ControlInstruction::Deallocate => {
+                let e = self.e;
+
+                let num_frame_e = self.and_stack.top().unwrap().global_index;
+                let num_frame_b = self.or_stack
+                                      .top()
+                                      .map(|fr| fr.global_index)
+                                      .unwrap_or(0);
+
+                self.p = self.and_stack[e].cp;
+                self.e = self.and_stack[e].e;
+
+                if num_frame_e > num_frame_b {
+                    let top_e = self.and_stack.top().unwrap().e;
+                    self.and_stack.drop_frames(top_e - self.e + 1);
+                }
+            },
+            &ControlInstruction::Proceed =>
+                self.p = self.cp,
+        };
+    }
+
+    fn execute_choice_instr(&mut self, instr: &ChoiceInstruction)
+    {
+        match instr {
+            &ChoiceInstruction::TryMeElse(offset) => {
+                let n = self.num_of_args;
+                let num_frames = self.num_frames();
+
+                self.or_stack.push(num_frames + 1,
+                                   self.e,
+                                   self.cp,
+                                   self.b,
+                                   self.p + offset,
+                                   self.tr,
+                                   self.h,
+                                   self.num_of_args);
+
+                self.b = self.or_stack.len() - 1;
+                let b = self.b;
+
+                for i in 1 .. n + 1 {
+                    self.or_stack[b][i] = self.registers[i].clone();
+                }
+
+                self.hb = self.h;
+                self.p += 1;
+            },
+            &ChoiceInstruction::RetryMeElse(offset) => {
+                let b = self.b;
+                let n = self.or_stack[b].num_args();
+
+                for i in 1 .. n + 1 {
+                    self.registers[i] = self.or_stack[b][i].clone();
+                }
+
+                self.e = self.or_stack[b].e;
+                self.cp = self.or_stack[b].cp;
+
+                self.or_stack[b].bp = self.p + offset;
+
+                let old_tr  = self.or_stack[b].tr;
+                let curr_tr = self.tr;
+
+                self.unwind_trail(old_tr, curr_tr);
+                self.tr = self.or_stack[b].tr;
+
+                self.trail.truncate(self.tr);
+                self.heap.truncate(self.or_stack[b].h);
+
+                self.h  = self.or_stack[b].h;
+                self.hb = self.h;
+
+                self.p += 1;
+            },
+            &ChoiceInstruction::TrustMe => {
+                let b = self.b;
+                let n = self.or_stack[b].num_args();
+
+                for i in 1 .. n + 1 {
+                    self.registers[i] = self.or_stack[b][i].clone();
+                }
+
+                self.e  = self.or_stack[b].e;
+                self.cp = self.or_stack[b].cp;
+
+                let old_tr  = self.or_stack[b].tr;
+                let curr_tr = self.tr;
+
+                self.unwind_trail(old_tr, curr_tr);
+
+                self.tr = self.or_stack[b].tr;
+                self.trail.truncate(self.tr);
+
+                self.h = self.or_stack[b].h;
+                self.heap.truncate(self.h);
+
+                self.b = self.or_stack[b].b;
+
+                self.or_stack.pop();
+
+                self.hb = self.h;
+                self.p += 1;
+            }
+        }
+    }
+
+    fn reset(&mut self) {
+        self.h = 0;
+        self.hb = 0;
+        self.e = 0;
+        self.b = 0;
+        self.s = 0;
+        self.tr = 0;
+        self.p = CodePtr::TopLevel;
+        self.cp = CodePtr::TopLevel;
+        self.num_of_args = 0;
+
+        self.fail = false;
+        self.trail.clear();
+        self.heap.clear();
+        self.mode = MachineMode::Write;
+        self.and_stack.clear();
+        self.or_stack.clear();
+        self.registers = vec![Addr::HeapCell(0); 64];
+    }
+}

src/prolog/mod.rs

@@ -0,0 +1,9 @@
+pub mod and_stack;
+pub mod ast;
+pub mod codegen;
+pub mod heapview;
+pub mod io;
+pub mod iterators;
+pub mod prolog_parser;
+pub mod machine;
+pub mod or_stack;

src/prolog/or_stack.rs

@@ -0,0 +1,112 @@
+use prolog::ast::*;
+
+use std::ops::{Index, IndexMut};
+use std::vec::Vec;
+
+pub struct Frame {
+    pub global_index: usize,
+    pub e: usize,
+    pub cp: CodePtr,
+    pub b: usize,
+    pub bp: CodePtr,
+    pub tr: usize,
+    pub h: usize,
+    args: Vec<Addr>
+}
+
+impl Frame {
+    fn new(global_index: usize,
+           e: usize,
+           cp: CodePtr,
+           b: usize,
+           bp: CodePtr,
+           tr: usize,
+           h: usize,
+           n: usize)
+           -> Self
+    {
+        Frame {
+            global_index: global_index,
+            e: e,
+            cp: cp,
+            b: b,
+            bp: bp,
+            tr: tr,
+            h: h,
+            args: vec![Addr::HeapCell(0); n]
+        }
+    }
+
+    pub fn num_args(&self) -> usize {
+        self.args.len()
+    }
+}
+
+pub struct OrStack(Vec<Frame>);
+
+impl OrStack {
+    pub fn new() -> Self {
+        OrStack(Vec::new())
+    }
+
+    pub fn push(&mut self,
+                global_index: usize,
+                e: usize,
+                cp: CodePtr,
+                b: usize,
+                bp: CodePtr,
+                tr: usize,
+                h: usize,
+                n: usize)
+    {
+        self.0.push(Frame::new(global_index, e, cp, b, bp, tr, h, n));
+    }
+
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    pub fn clear(&mut self) {
+        self.0.clear()
+    }
+
+    pub fn top(&self) -> Option<&Frame> {
+        self.0.last()
+    }
+
+    pub fn pop(&mut self) {
+        self.0.pop();
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
+
+impl Index<usize> for OrStack {
+    type Output = Frame;
+
+    fn index(&self, index: usize) -> &Self::Output {
+        self.0.index(index)
+    }
+}
+
+impl IndexMut<usize> for OrStack {
+    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
+        self.0.index_mut(index)
+    }
+}
+
+impl Index<usize> for Frame {
+    type Output = Addr;
+
+    fn index(&self, index: usize) -> &Self::Output {
+        self.args.index(index - 1)
+    }
+}
+
+impl IndexMut<usize> for Frame {
+    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
+        self.args.index_mut(index - 1)
+    }
+}

src/prolog/prolog_parser.lalrpop

@@ -0,0 +1,77 @@
+use prolog::ast::*;
+
+use std::cell::Cell;
+
+grammar;
+
+pub TopLevel: TopLevel = {
+    "?-" <t:Term> "." => TopLevel::Query(t),
+    <Predicate> => TopLevel::Predicate(<>),
+    <Rule>  "." => TopLevel::Rule(<>),
+    <Term>  "." => TopLevel::Fact(<>)
+};
+
+Atom : Atom = {
+    r"[a-z][a-z0-9_]*" => <>.trim().to_string(),
+};
+
+BoxedTerm : Box<Term> = {
+    <t:Term> => Box::new(t)
+};
+
+Clause : Term = {
+    <a:Atom> "(" <ts: (<BoxedTerm> ",")*> <t:BoxedTerm> ")" => {
+     	let mut ts = ts;
+     	ts.push(t);
+	Term::Clause(Cell::default(), a, ts)
+    }
+};
+
+List : Term = {
+    "[]" => Term::Constant(Cell::default(), Constant::EmptyList),
+    "[" <ListInternals> "]" => <>
+};
+
+ListInternals : Term = {
+    <t:BoxedTerm> => Term::Cons(Cell::default(),
+                                t,
+				Box::new(Term::Constant(Cell::default(),
+				                        Constant::EmptyList))),
+    <t:BoxedTerm>  "," <li: ListInternals> => Term::Cons(Cell::default(),
+                                                         t,
+							 Box::new(li)),
+    <t1:BoxedTerm> "|" <t2:BoxedTerm> => Term::Cons(Cell::default(), t1, t2)
+};
+
+Predicate : Vec<PredicateClause> = {
+    <pcs: (<PredicateClause>)+> <pc: PredicateClause> => {
+        let mut pcs = pcs;
+	pcs.push(pc);
+	pcs
+    }
+};
+
+PredicateClause : PredicateClause = {
+    <Rule> "." => PredicateClause::Rule(<>),
+    <Term> "." => PredicateClause::Fact(<>)
+};
+
+Rule : Rule = {
+    <c:Clause> ":-" <h:Term> <cs: ("," <Term>)*> =>
+        Rule { head: (c, h), clauses: cs },
+    <a:Atom> ":-" <h:Term> <cs: ("," <Term>)*> =>
+        Rule { head: (Term::Constant(Cell::default(), Constant::Atom(a)), h),
+               clauses: cs }
+};
+
+Term : Term = {
+    <Atom>   => Term::Constant(Cell::default(), Constant::Atom(<>)),
+    <Clause> => <>,
+    <List>   => <>,
+    <Var>    => Term::Var(Cell::default(), <>),
+    "_"      => Term::AnonVar
+};
+
+Var : Var = {
+    r"[A-Z][a-z0-9_]*" => <>.trim().to_string()
+};