[m-users.] A predicate taking a lists of predicates as input?

[Volker Wysk]

Hi, Anders

You have two errors:

1. You don't need and can't have the TNKS argument to your ta_rule inst,
since it isn't used on the right side.

2. You need to specify a higher order inst matching your higher order type,
for both predicates.

Do it like this:

:- type ta_rule(TKNS) ==  pred(TKNS, TKNS). % 'ta' abbreviates 'type alias'
:- inst ta_rule       == (pred(in,   out) is semidet).

:- pred ?(list(ta_rule(TKNS)), TKNS, TKNS).
:- mode ?(in(list(ta_rule)),   in,   out).

:- pred apply_rules(list(ta_rule(TKNS)), TKNS, TKNS).
:- mode apply_rules(in(list(ta_rule)),   in,   out) is semidet.

Cheers,
Volker

Am Sonntag, dem 21.09.2025 um 18:28 +0200 schrieb Anders Lundstedt:
> Dear all,
> 
> 
> For the purpose of a parser I am writing, I want to have a DCG rule ?
> taking a list of DCG rules as input and that then tries to apply all
> rules in the list.
> 
> 
> GitHub Gist of the following:
> 
> https://gist.github.com/anderslundstedt/1f5b1d8ad6b4d2404dae4a602d3d920d
> 
> 
> My attempt:
> 
> :- type ta_rule(TKNS) ==  pred(TKNS, TKNS). % 'ta' abbreviates 'type
> alias'
> :- inst ta_rule(TKNS) == (pred(in,   out) is semidet).
> 
> :- pred ?(list(ta_rule(TKNS)), TKNS, TKNS).
> :- mode ?(in,                  in,   out).
> ?(RULES) --> (
>   apply_rules(RULES) -> {true};
>                         {true}
> ).
> 
> :- pred apply_rules(list(ta_rule(TKNS)), TKNS, TKNS).
> :- mode apply_rules(in,                  in,   out) is semidet.
> apply_rules([])     --> {true}.
> apply_rules([R|RS]) --> R, apply_rules(RS).
> 
> 
> Compilation error:
> 
> In clause for `apply_rules(in, in, out)':
>   in argument 1 (i.e. the predicate term) of higher-order predicate
>   call:
>   mode error: variable `R' has instantiatedness `ground',
>   expecting higher-order pred inst of arity 2.
> 
> 
> Any advice would be greatly appreciated!
> 
> 
> 
> Best,
> 
> Anders Lundstedt
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