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correct can_be/2 for partial lists #57

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Sep 28, 2018
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correct can_be/2 for partial lists #57

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21 changes: 14 additions & 7 deletions src/prolog/lib/error.pl
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
:- module(error, [must_be/2, can_be/2]).
:- module(error, [must_be/2,
can_be/2]).

/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Written September 2018 by Markus Triska ([email protected])
Expand Down Expand Up @@ -47,16 +48,13 @@
ilist([_|Ls]) :- ilist(Ls).



/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
can_be(Type, Term)

This predicate is intended for type-checks of built-in predicates.

It asserts that:

1) Term is either a variable or instantiated *and*
2) _if_ it is instantiated, then it is an instance of Type.
It asserts that there is a substitution which, if applied to Term,
makes it an instance of Type.

It corresponds to usage mode ?Term.

Expand All @@ -66,9 +64,18 @@

can_be(Type, Term) :-
( var(Term) -> true
; must_be(Type, Term)
; can_(Type, Term) -> true
; type_error(Type, Term)
).

can_(integer, Term) :- integer(Term).
can_(atom, Term) :- atom(Term).
can_(list, Term) :- list_or_partial_list(Term).

list_or_partial_list(Var) :- var(Var).
list_or_partial_list([]).
list_or_partial_list([_|Ls]) :-
list_or_partial_list(Ls).

/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Shorthands for throwing ISO errors.
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