higher-order DCG rule exported from module

[rodrigosetti]

I'm attempting to export a higher-order DCG rule from a module:

% test.pl
:- module(test, [p//1]).

p(G_0) --> G_0.

?- [test].
   true.
?- phrase(p("hello"), R).
   error(existence_error(procedure,phrase/3),phrase/3).

I don't understand what the error means.

I attempted to use meta_predicate directive but no mode seems to work. Maybe Scryer can add support for SWI's // mode?

[triska]

Scryer Prolog transforms the DCG you wrote to the following Prolog predicate:

p(A,B,C) :- phrase(A,B,C).

However, phrase/3 is not defined in your module, and so you get an existence error when invoking the DCG.

You can import library(dcgs) to make phrase/3 available in your code, by adding the directive:

:- use_module(library(dcgs)).

You can also simply use phrase/2 or phrase/3 directly:

$ scryer-prolog
?- use_module(library(dcgs)).
   true.
?- phrase("hello", Ls).
   Ls = "hello".

[rodrigosetti]

Thank you for your reply, Markus - this is was a huge overlook from my part :)

[rodrigosetti]

Shouldn't -->/2 expansion be defined in the dcgs library? (instead of being built-in in the compiler) that would save some confusion from beginners like me.

[triska]

Yes, absolutely! In fact, I also filed this as an issue, please see the discussion and the explanation for the current state: #987