[z5h]

> How do you debug DCGs? I get "false." instead of "syntax error at line 23", which is unacceptable for bigger inputs.

I also sympathize. "false" as the default failure mode is a challenge with Prolog. Most Prologs I've used have good debugging/stepping features (see spy and trace predicates), logical debugging of pure monotonic Prolog can often help (explained by Markus Triska), you can easily write (use existing) meta predicates that assert a called predicate must not fail otherwise throw an exception. For example: here the ./ is supposed to look like a checkmark. So `./ true.` is true. `./ false` throws an exception.

  :- op(920, fy, './').
  :- meta_predicate './'(0).
  './'(X) :- call(X) *-> true ; prolog_debug:assertion_failed(fail, X).

[upghost]

Ahh nice, I like this one!