0dc94ecc0b
For a recent example that would benefit from this, see "Prolog Basics
Explained with Pokémon" by @alexpetros:
https://unplannedobsolescence.com/blog/prolog-basics-pokemon/
Thank you a lot!
142 lines
3.3 KiB
Prolog
142 lines
3.3 KiB
Prolog
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/** Safe type tests.
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"si" stands for "sufficiently instantiated". It can also be read as
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"safe inference", so possibly also other predicates are candidates
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for this library.
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A safe type test:
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- throws an *instantiation error* if the argument is
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not sufficiently instantiated to make a sound decision
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- *succeeds* if the argument is of the specified type
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- *fails* otherwise.
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For instance, `atom_si(A)` yields an *instantiation error* if `A` is a
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variable. This is logically sound, since in that case the argument
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is not sufficiently instantiated to make any decision.
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The definitions are taken from [Safer type tests in Prolog](https://stackoverflow.com/questions/27306453/safer-type-tests-in-prolog).
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Examples:
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```
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?- chars_si(Cs).
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error(instantiation_error,list_si/1).
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?- chars_si([h|Cs]).
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error(instantiation_error,list_si/1).
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?- chars_si("hello").
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true.
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?- chars_si(hello).
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false.
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```
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*/
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:- module(si, [atom_si/1,
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integer_si/1,
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atomic_si/1,
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list_si/1,
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character_si/1,
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term_si/1,
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chars_si/1,
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dif_si/2,
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not_si/1,
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when_si/2]).
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:- use_module(library(lists)).
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atom_si(A) :-
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functor(A, _, 0), % for the instantiation error
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atom(A).
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integer_si(I) :-
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functor(I, _, 0),
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integer(I).
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atomic_si(AC) :-
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functor(AC,_,0).
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% list_si(L) :-
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% \+ \+ length(L, _),
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% sort(L, _).
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list_si(L0) :-
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'$skip_max_list'(_,_, L0,L),
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( nonvar(L) -> L = []
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; throw(error(instantiation_error, list_si/1))
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).
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character_si(Ch) :-
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functor(Ch,Ch,0),
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atom(Ch),
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atom_length(Ch,1).
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term_si(Term) :-
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( ground(Term) -> acyclic_term(Term)
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; throw(error(instantiation_error, term_si/1))
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).
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chars_si(Chs0) :-
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'$skip_max_list'(_,_, Chs0,Chs),
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( nonvar(Chs) -> Chs == [] ; true ), % fails for infinite lists too
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failnochars(Chs0, Uninstantiated),
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( nonvar(Uninstantiated)
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-> throw(error(instantiation_error, chars_si/1))
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; true
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).
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failnochars(Chs0, U) :-
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( var(Chs0) -> U = true
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; Chs0 == [] -> true
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; Chs0 = [Ch|Chs1],
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( nonvar(Ch) -> atom(Ch), atom_length(Ch,1)
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; U = true
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),
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failnochars(Chs1, U)
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).
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dif_si(X, Y) :-
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X \== Y,
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( X \= Y -> true
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; throw(error(instantiation_error,dif_si/2))
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).
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%% not_si(+Goal).
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%
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% True if Goal is not provable. Instantiation error if Goal is not
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% ground.
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:- meta_predicate(not_si(0)).
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not_si(Goal) :-
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term_si(Goal),
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\+ Goal.
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:- meta_predicate(when_si(+, 0)).
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%% when_si(Condition, Goal).
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%
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% Executes Goal when Condition becomes true. Throws an instantiation error if
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% it can't decide.
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when_si(Condition, Goal) :-
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% Taken from https://stackoverflow.com/a/40449516
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( when_condition_si(Condition) ->
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( Condition ->
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Goal
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; throw(error(instantiation_error,when_si/2))
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)
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; throw(error(domain_error(when_condition_si, Condition),_))
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).
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when_condition_si(Cond) :-
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var(Cond), !, throw(error(instantiation_error,when_condition_si/2)).
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when_condition_si(ground(_)).
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when_condition_si(nonvar(_)).
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when_condition_si((A, B)) :-
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when_condition_si(A),
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when_condition_si(B).
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when_condition_si((A ; B)) :-
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when_condition_si(A),
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when_condition_si(B).
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