81 lines
1.9 KiB
Prolog
81 lines
1.9 KiB
Prolog
/**
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Provides predicate `dif/2`. `dif/2` is a constraint that is true only if both of its
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arguments are different terms.
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*/
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:- module(dif, [dif/2]).
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:- use_module(library(atts)).
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:- use_module(library(dcgs)).
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:- use_module(library(lists), [append/3]).
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:- attribute dif/1.
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put_dif_att(Var, X, Y) :-
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( get_atts(Var, +dif(Z)) ->
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sort([X \== Y | Z], NewZ),
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put_atts(Var, +dif(NewZ))
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; put_atts(Var, +dif([X \== Y]))
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).
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dif_set_variables([], _, _).
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dif_set_variables([Var|Vars], X, Y) :-
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put_dif_att(Var, X, Y),
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dif_set_variables(Vars, X, Y).
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append_goals([], _).
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append_goals([Var|Vars], Goals) :-
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( get_atts(Var, +dif(VarGoals)) ->
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append(Goals, VarGoals, NewGoals0),
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sort(NewGoals0, NewGoals)
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; NewGoals = Goals
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),
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put_atts(Var, +dif(NewGoals)),
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append_goals(Vars, Goals).
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verify_attributes(Var, Value, Goals) :-
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( get_atts(Var, +dif(Goals)) ->
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term_variables(Value, ValueVars),
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append_goals(ValueVars, Goals)
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; Goals = []
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).
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% Probably the world's worst dif/2 implementation. I'm open to
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% suggestions for improvement.
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%% dif(?X, ?Y).
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%
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% True iff X and Y are different terms. Unlike `\=/2`, `dif/2` is more declarative because if X and Y can
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% unify but they're not yet equal, the decision is delayed, and prevents X and Y to become equal later.
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% Examples:
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%
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% ```
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% ?- dif(a, a).
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% false.
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% ?- dif(a, b).
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% true.
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% ?- dif(X, b).
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% dif:dif(X,b).
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% ?- dif(X, b), X = b.
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% false.
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% ```
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dif(X, Y) :-
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X \== Y,
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( X \= Y -> true
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; ( term_variables(X, XVars),
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term_variables(Y, YVars),
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dif_set_variables(XVars, X, Y),
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dif_set_variables(YVars, X, Y)
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)
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).
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gather_dif_goals([]) --> [].
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gather_dif_goals([(X \== Y) | Goals]) -->
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[dif:dif(X, Y)],
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gather_dif_goals(Goals).
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attribute_goals(X) -->
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{ get_atts(X, +dif(Goals)) },
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gather_dif_goals(Goals),
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{ put_atts(X, -dif(_)) }.
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