Howto import operators in swi-prolog?

Question

I have the following file:

:- use_module(library(clpfd)).

isPowTwo(N) :- N #> 0, N #= 2^_.

Everything works fine:

?- [importTest].
%     library(pairs) compiled into pairs 0.00 sec, 22 clauses
%    library(lists) compiled into lists 0.01 sec, 122 clauses
%    library(occurs) compiled into occurs 0.00 sec, 14 clauses
%   library(apply_macros) compiled into apply_macros 0.01 sec, 168 clauses
%   library(assoc) compiled into assoc 0.01 sec, 103 clauses
%  library(clpfd) compiled into clpfd 0.15 sec, 2,808 clauses
% importTest compiled 0.16 sec, 2,813 clauses
true.

Now I want to import just the two used operators:

:- use_module(library(clpfd), [(#>)/2, (#=)/2]).

isPowTwo(N) :- N #> 0, N #= 2^_.

And it does not work:

?- [importTest].
%     library(pairs) compiled into pairs 0.00 sec, 22 clauses
%    library(lists) compiled into lists 0.01 sec, 122 clauses
%    library(occurs) compiled into occurs 0.00 sec, 14 clauses
%   library(apply_macros) compiled into apply_macros 0.01 sec, 168 clauses
%   library(assoc) compiled into assoc 0.01 sec, 103 clauses
%  library(clpfd) compiled into clpfd 0.16 sec, 2,808 clauses
ERROR: .../importTest.pl:3:17: Syntax error: Operator expected
% importTest compiled 0.16 sec, 2,812 clauses
true.

?- isPowTwo(1).
ERROR: toplevel: Undefined procedure: isPowTwo/1 (DWIM could not correct goal)

Leaving the parentheses around #> and #= makes no difference.

Paulo Moura Paulo Moura · Accepted Answer · 2016-09-05T12:58:40

You have a syntax error in your use_module/2 directive (hence the Syntax error: Operator expected error messages). You can correct it by writing:

:- use_module(library(clpfd), [(#>)/2, (#=)/2]).

But you will also need to import the corresponding operators for use in the definition of your isPowTwo/1 predicate:

:- use_module(library(clpfd), [(#>)/2, (#=)/2, op(700,xfx,(#>)), op(700,xfx,(#=))]).

Howto import operators in swi-prolog?

1 Answers