Merge pull request #2218 from bakaq/when

Add when/2 and when_si/2

src/lib/si.pl

@@ -39,7 +39,8 @@
                character_si/1,
                term_si/1,
                chars_si/1,
-               dif_si/2]).
+               dif_si/2,
+               when_si/2]).
 
 :- use_module(library(lists)).
 
@@ -98,3 +99,31 @@ dif_si(X, Y) :-
    ( X \= Y -> true
    ; throw(error(instantiation_error,dif_si/2))
    ).
+
+:- meta_predicate(when_si(+, 0)).
+
+%% when_si(Condition, Goal).
+%
+% Executes Goal when Condition becomes true. Throws an instantiation error if
+% it can't decide.
+when_si(Condition, Goal) :-
+    % Taken from https://stackoverflow.com/a/40449516
+    (   when_condition_si(Condition) ->
+        (   Condition ->
+            Goal
+        ;   throw(error(instantiation_error,when_si/2))
+        )
+    ;   throw(error(domain_error(when_condition_si, Condition),_))
+    ).
+
+when_condition_si(Cond) :-
+    var(Cond), !, throw(error(instantiation_error,when_condition_si/2)).
+when_condition_si(ground(_)).
+when_condition_si(nonvar(_)).
+when_condition_si((A, B)) :-
+    when_condition_si(A),
+    when_condition_si(B).
+when_condition_si((A ; B)) :-
+    when_condition_si(A),
+    when_condition_si(B).
+

src/lib/when.pl

@@ -0,0 +1,106 @@
+/**
+Provides the predicate `when/2`.
+*/
+
+:- module(when, [when/2]).
+
+:- use_module(library(atts)).
+:- use_module(library(dcgs)).
+:- use_module(library(lists)).
+:- use_module(library(lambda)).
+
+:- use_module(library(format)).
+:- use_module(library(debug)).
+
+:- attribute when_list/1.
+
+:- meta_predicate(when(+, 0)).
+
+%% when(Condition, Goal).
+%
+% Executes Goal when Condition becomes true.
+when(Condition, Goal) :-
+    (   when_condition(Condition) ->
+        (   Condition ->
+            Goal
+        ;   term_variables(Condition, Vars),
+            maplist(
+                [Goal, Condition]+\Var^(
+                    get_atts(Var, when_list(Whens0)) ->
+                    Whens = [when(Condition, Goal) | Whens0],
+                    put_atts(Var, when_list(Whens))
+                ;   put_atts(Var, when_list([when(Condition, Goal)]))
+                ),
+                Vars
+            )
+        )
+    ;   throw(error(domain_error(when_condition, Condition),_))
+    ).
+
+when_condition(Cond) :-
+    % Should this be delayed?
+    var(Cond), !, throw(error(instantiation_error,when_condition/1)).
+when_condition(ground(_)).
+when_condition(nonvar(_)).
+when_condition((A, B)) :-
+    when_condition(A),
+    when_condition(B).
+when_condition((A ; B)) :-
+    when_condition(A),
+    when_condition(B).
+
+remove_goal([], _, []).
+remove_goal([G0|G0s], Goal, Goals) :-
+    (   G0 == Goal ->
+        remove_goal(G0s, Goal, Goals)
+    ;   Goals = [G0|Goals1],
+        remove_goal(G0s, Goal, Goals1)
+    ).
+
+vars_remove_goal(Vars, Goal) :-
+    maplist(
+        Goal+\Var^(
+            get_atts(Var, when_list(Whens0)) ->
+            remove_goal(Whens0, Goal, Whens),
+            (   Whens = [] ->
+                put_atts(Var, -when_list(_))
+            ;   put_atts(Var, when_list(Whens))
+            )
+        ;   true
+        ),
+        Vars
+    ).
+
+reinforce_goal(Goal0, Goal) :-
+    Goal = (
+        term_variables(Goal0, Vars),
+        when:vars_remove_goal(Vars, Goal0),
+        Goal0
+    ).
+
+verify_attributes(Var, Value, Goals) :-
+    (   get_atts(Var, when_list(Whens)) ->
+        (   var(Value) ->
+            (   get_atts(Value, when_list(WhensValue)) ->
+                append(Whens, WhensValue, WhensNew),
+                put_atts(Value, when_list(WhensNew))
+            ;   put_atts(Value, when_list(Whens))
+            ),
+            Goals = []
+        ;   maplist(reinforce_goal, Whens, Goals)
+        )
+    ;   Goals = []
+    ).
+
+gather_when_goals([], _) --> [].
+gather_when_goals([When|Whens], Var) -->
+    ( { term_variables(When, [V0|_]), Var == V0 } ->
+        [when:When]
+    ;   []
+    ),
+    gather_when_goals(Whens, Var).
+
+attribute_goals(Var) -->
+    { get_atts(Var, when_list(Whens)) },
+    gather_when_goals(Whens, Var),
+    { put_atts(Var, -when_list(_)) }.

src/tests/when.pl

@@ -0,0 +1,145 @@
+/**/
+
+:- use_module(library(format)).
+:- use_module(library(dcgs)).
+:- use_module(library(lists)).
+:- use_module(library(debug)).
+:- use_module(library(atts)).
+
+:- use_module(library(when)).
+
+test("condition true before ground/1",(
+    A = 1,
+    when(ground(A), Run = true),
+    Run == true
+)).
+
+test("condition true before nonvar/1",(
+    A = a(_),
+    when(nonvar(A), Run = true),
+    Run == true
+)).
+
+test("condition true before ','/2",(
+    A = 1,
+    B = a(_),
+    when((ground(A), nonvar(B)), Run = true),
+    Run == true
+)).
+
+test("condition true before (;)/2",(
+    A = 1,
+    when((ground(A) ; nonvar(_)), Run1 = true),
+    Run1 == true,
+
+    B = a(_),
+    when((ground(_) ; nonvar(B)), Run2 = true),
+    Run2 == true
+)).
+
+test("condition true after ground/1",(
+    when(ground(A), Run = true),
+    var(Run),
+    A = 1,
+    Run == true
+)).
+
+test("condition true after nonvar/1",(
+    when(nonvar(A), Run = true),
+    var(Run),
+    A = a(_),
+    Run == true
+)).
+
+test("condition true after ','/2",(
+    when((ground(A), nonvar(B)), Run = true),
+    var(Run),
+    A = 1,
+    var(Run),
+    B = a(_),
+    Run == true
+)).
+
+test("condition true after (;)/2",(
+    when((ground(A) ; nonvar(_)), Run1 = true),
+    var(Run1),
+    A = 1,
+    Run1 == true,
+
+    when((ground(_) ; nonvar(B)), Run2 = true),
+    var(Run2),
+    B = a(_),
+    Run2 == true
+)).
+
+test("multiple when/2 on same variable",(
+    when(nonvar(A), Run1 = true),
+    when(ground(A), Run2 = true),
+    var(Run1), var(Run2),
+    A = a(B),
+    Run1 == true, var(Run2),
+    B = 1,
+    Run2 == true
+)).
+
+main :-
+    findall(test(Name, Goal), test(Name, Goal), Tests),
+    run_tests(Tests, Failed),
+    show_failed(Failed),
+    halt.
+
+main_quiet :-
+    findall(test(Name, Goal), test(Name, Goal), Tests),
+    run_tests_quiet(Tests, Failed),
+    (   Failed = [] ->
+        format("All tests passed", [])
+    ;   format("Some tests failed", [])
+    ),
+    halt.
+
+portray_failed_([]) --> [].
+portray_failed_([F|Fs]) -->
+    "\"", F, "\"",  "\n", portray_failed_(Fs).
+
+portray_failed([]) --> [].
+portray_failed([F|Fs]) -->
+    "\n", "Failed tests:", "\n", portray_failed_([F|Fs]).
+
+show_failed(Failed) :-
+    phrase(portray_failed(Failed), F),
+    format("~s", [F]).
+
+run_tests([], []).
+run_tests([test(Name, Goal)|Tests], Failed) :-
+    format("Running test \"~s\"~n", [Name]),
+    (   call(Goal) ->
+        Failed = Failed1
+    ;   format("Failed test \"~s\"~n", [Name]),
+        Failed = [Name|Failed1]
+    ),
+    run_tests(Tests, Failed1).
+
+run_tests_quiet([], []).
+run_tests_quiet([test(Name, Goal)|Tests], Failed) :-
+    (   call(Goal) ->
+        Failed = Failed1
+    ;   Failed = [Name|Failed1]
+    ),
+    run_tests_quiet(Tests, Failed1).
+
+assert_p(A, B) :-
+    phrase(portray_clause_(A), Portrayed),
+    phrase((B, ".\n"), Portrayed).
+
+call_residual_goals(Goal, ResidualGoals) :-
+    call_residue_vars(Goal, Vars),
+    variables_residual_goals(Vars, ResidualGoals).
+
+variables_residual_goals(Vars, Goals) :-
+    phrase(variables_residual_goals(Vars), Goals).
+
+variables_residual_goals([]) --> [].
+variables_residual_goals([Var|Vars]) -->
+    dif_:attribute_goals(Var),
+    variables_residual_goals(Vars).
+