[m-users.] Using type constructors in predicate heads

[Sean Charles (emacstheviking)]

Wow, that obvious! LOL    I’ve been working on something close but not close enough to get it past the compiler yet hahaha.
Thanks a lot for that, I can have  read after I’ve done the dishes… meh.

Cheers
Sean!


> On 7 Nov 2021, at 09:24, Julien Fischer <jfischer at opturion.com> wrote:
> 
> 
> Hi Sean,
> 
> On Sun, 7 Nov 2021, Sean Charles (emacstheviking) wrote:
> 
>> I made your suggested change, it works fine but… can I push it a
>> little further? I went back to the Mercury crash course page as it
>> mentioned non_empty_list, I found no references in the reference guide
>> or the library but plenty in the source code for my ROTD version… but
>> I am unsure of the syntax, if possible, of stating that the the list
>> of terms to the sexp() functor is guaranteed to be non empty because:
>>     ( if Term = sexp(_, [ tk(_,F) | _ ]) then
>> already established that fact prior to the call…but this leads me to
>> my final question: —should— I be doing this? I ask because
>> :- mode do_gencall(in, renderer, in(sexp), in, out) is det.
>> do_gencall(_L, R, sexp(_, [Functor|Args]), !T) :-
>> This obv. fails because the unification into head and tail could fail,
>> but if I declare it as semidet then obv. I have to check the call
>> further up the code, again I totally understand why but my question
>> then is, should I do this at all ? I have not used Haskell in a while
>> now, plumping for Mercury, but I was getting reasonably proficient at
>> the concept of `programming with types` and this feels similar. If I
>> instructed the compiler that the mode was something like
>> :- mode do_gencall(in, renderer, in(sexp-with-non-empty-args), in, out) is det.
>> then presumably the predicate could remain as `det`, everybody is
>> happy and my code feels more rigorous for it. I refer to the sample of
>> code in the file ‘./mercury-srcdist-20.06.1/browser/parse.m’ , lines
>> 305 to 307:
>> :- pred lexer_arg(list(char)::in(non_empty_list), list(token)::out) is det.
>> lexer_arg([Head | Tail], Toks) :-
>> This clearly shows to me that by using non_empty_list the compiler is
>> happy to let me deconstruct the list in the clause head with no
>> problems. I am not sure how to combine the mode you showed me with the
>> addition of the non empty constraint as well that’s all!
> 
> As a separate named inst:
> 
>   :- inst sexp_with_non_empty_args for snode/0
>       --->    sexp(ground, non_empty_list).
> 
>   :- mode do_gencall(in, renderer, in(sexp_with_non_empty_args),
>      in, out) is det.
> 
> or inline:
> 
>   :- mode do_gencall(in, renderer, in(bound(sexp(ground, non_empty_list))),
>      in, out) is det.
> 
> Julien