What does syntax_error(incomplete_reduction) mean when appearing from a call of phrase/2?

[razetime]

A minimal example:

?- [user].

cube(Num) --> seq(StrNum), { number_chars(Num, StrNum) }.

?- phrase(cube(X),"1").
   error(syntax_error(incomplete_reduction),number_chars/2:0).
?- phrase(cube(X),"1",Y).
   error(syntax_error(incomplete_reduction),number_chars/2:0).

I'd like to know how to fix errors like this, that come from within metapredicates, and how best to go about debugging them and finding what's wrong with my logic.

[mthom]

Top of my head, seq first unifies StrNum to []. number_chars/2 throws a syntax error because [] can't be parsed to a number.

[triska]

Regarding debugging: Since we know the error arises from number_chars/2, we can use ($)/1 from library(debug) to get more information about how the predicate is invoked. Simply prepend $ to the goal:

:- use_module(library(debug)).
:- use_module(library(dcgs)).

cube(N) --> seq(Cs), { $ number_chars(N, Cs) }.

Yielding:

?- phrase(cube(X),"1").
call:number_chars(A,[]).
exception:error(syntax_error(incomplete_reduction),number_chars/2:0):number_chars(A,[]).

So, yes, as @mthom already mentioned, the empty list [] has no characters that can be parsed to a number:

?- number_chars(A,[]).
   error(syntax_error(incomplete_reduction),number_chars/2:0).