Revert "remove Term"

This reverts commit 3b5879841aedecba5057c70c71da0ba23e5cd84a.

src/types.rs

@@ -7,6 +7,7 @@ use crate::machine::heap::*;
 use crate::machine::machine_indices::*;
 use crate::machine::streams::*;
 use crate::parser::ast::Fixnum;
+use crate::parser::ast::Literal;
 
 use std::cmp::Ordering;
 use std::convert::TryFrom;
@@ -14,6 +15,8 @@ use std::fmt;
 use std::mem;
 use std::ops::{Add, Sub, SubAssign};
 
+use dashu::{Integer, Rational};
+
 #[derive(BitfieldSpecifier, Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
 #[repr(u8)]
 #[bits = 6]
@@ -307,6 +310,67 @@ impl fmt::Debug for HeapCellValue {
     }
 }
 
+impl From<Literal> for HeapCellValue {
+    #[inline]
+    fn from(literal: Literal) -> Self {
+        match literal {
+            Literal::Atom(name) => atom_as_cell!(name),
+            Literal::CodeIndex(ptr) => {
+                untyped_arena_ptr_as_cell!(UntypedArenaPtr::from(ptr))
+            }
+            Literal::Fixnum(n) => fixnum_as_cell!(n),
+            Literal::Integer(bigint_ptr) => {
+                typed_arena_ptr_as_cell!(bigint_ptr)
+            }
+            Literal::Rational(bigint_ptr) => {
+                typed_arena_ptr_as_cell!(bigint_ptr)
+            }
+            Literal::Float(f) => HeapCellValue::from(f.as_ptr()),
+        }
+    }
+}
+
+impl TryFrom<HeapCellValue> for Literal {
+    type Error = ();
+
+    fn try_from(value: HeapCellValue) -> Result<Literal, ()> {
+        read_heap_cell!(value,
+            (HeapCellValueTag::Atom, (name, arity)) => {
+                if arity == 0 {
+                    Ok(Literal::Atom(name))
+                } else {
+                    Err(())
+                }
+            }
+            (HeapCellValueTag::Fixnum, n) => {
+                Ok(Literal::Fixnum(n))
+            }
+            (HeapCellValueTag::F64, f) => {
+                Ok(Literal::Float(f.as_offset()))
+            }
+            (HeapCellValueTag::Cons, cons_ptr) => {
+                match_untyped_arena_ptr!(cons_ptr,
+                     (ArenaHeaderTag::Integer, n) => {
+                         Ok(Literal::Integer(n))
+                     }
+                     (ArenaHeaderTag::Rational, n) => {
+                         Ok(Literal::Rational(n))
+                     }
+                     (ArenaHeaderTag::IndexPtr, ip) => {
+                         Ok(Literal::CodeIndex(CodeIndex::from(ip)))
+                     }
+                     _ => {
+                         Err(())
+                     }
+                )
+            }
+            _ => {
+                Err(())
+            }
+        )
+    }
+}
+
 impl<T: ArenaAllocated> From<TypedArenaPtr<T>> for HeapCellValue
 where
     T::Payload: Sized,