Thoughts on testing DCG rules

[simonharris]

Grammar rules can be tested using plunit_assert. For example:

?- assert_output(phrase(position(Ordinal), ['the', 'last', 'digit'], []), [Ordinal], [third]).
[plunit_assert] Output does not match expected: expected [third], got [fourth]
false.

?- assert_output(phrase(position(Ordinal), ['the', 'last', 'digit'], []), [Ordinal], [fourth]).
Ordinal = fourth.

But the syntax is clunky and easy to mess up. Perhaps:

• detect it's a grammar rule rather than a predicate within an assert_* test predicate and transparently wrap the call in phrase/3
• treat them as explicitly different and begin a family of eg. dcg_assert_* tests (nb below)
• implement a DSL, something like position(['the', 'last', 'digit']) should_parse_to fourth.
• ...

nb.

assert_grammar_output(GrammarRule, Vars, Expected) :-
    phrase(GrammarRule, Input, []),
    GrammarRule =.. [Functor|Args],
    Vars = Args,
    Vars = Expected.