Merge pull request #164 from triska/master

ADDED: library(si) for safe type tests.

src/prolog/lib/si.pl

@@ -0,0 +1,47 @@
+/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+   Safe type tests
+   ===============
+
+   "si" stands for "sufficiently instantiated".
+
+   These predicates:
+
+    - throw instantiation errors if the argument is
+      not sufficiently instantiated to make a sound decision
+    - succeed if the argument is of the specified type
+    - fail otherwise.
+
+   For instance, atom_si(A) yields an *instantiation error* if A is a
+   variable. This is logically sound, since in that case the argument
+   is not sufficiently instantiated to make any decision.
+
+   The definitions are taken from:
+
+       https://stackoverflow.com/questions/27306453/safer-type-tests-in-prolog
+
+   "si" can also be read as "safe inference", so possibly also other
+   predicates are candidates for this library.
+- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
+
+:- module(si, [atom_si/1,
+               integer_si/1,
+               atomic_si/1,
+               list_si/1]).
+
+:- use_module(library(lists)).
+
+atom_si(A) :-
+   functor(A, _, 0),    % for the instantiation error
+   atom(A).
+
+integer_si(I) :-
+   functor(I, _, 0),
+   integer(I).
+
+atomic_si(AC) :-
+   functor(AC,_,0).
+
+list_si(L) :-
+   \+ \+ length(L, _),
+   sort(L, _).