Re: [m-users.] Insts of tuple types?

Volker Wysk <[email protected]> Mon, 07 Aug 2023 14:21:09 +0200

Newsgroups	gmane.comp.lang.mercury.general
Message-ID	<[email protected]>

Am Montag, dem 07.08.2023 um 17:38 +1000 schrieb Peter Wang:
> On Sun, 06 Aug 2023 19:37:53 +0200 Volker Wysk <[email protected]> wrote:
> > Hello.
> > 
> > I have an argument of this type:
> > 
> >     maybe(
> >         { (pred(io, io)),
> >           place_data
> >         }
> >     )
> > 
> > I need to specify a higher order inst because the first component is a
> > predicate. What is the inst? This inst doesn't work:
> > 
> >     :: out(
> >         maybe(
> >             { pred(di, io) is det,
> >               ground
> >             }
> >         ))
> > 
> > I get this error message:
> > 
> > amend.m:099: In declaration of predicate `amend_new_file_place'/5:
> > amend.m:099:   error: undefined inst `{}'/2.
> 
> It's written:
> 
>     bound({ ArgInst1, ArgInst2 })
> 
> which is the same as:
> 
>     bound('{}'( ArgInst1, ArgInst2 )
> 
> In a inst definition you can write:
> 
>     :- inst tuple
> 	--->	{ ArgInst1, ArgInst2 }.
> 
> which is the same as:
> 
>     :- inst tuple
> 	--->	'{}'( ArgInst1, ArgInst2 ).

I see now. This part was missing for my inst to work:

:- inst '{}'(I, J) == bound('{}'(I, J)).

Or:

:- inst '{}'(I,J) ---> '{}'(I,J).

> BTW, in case you haven't seen the chapter on Combined higher-order types
> and insts, you can define a type with a higher-order argument type with
> insts:
> 
>     :- type pred_wrapper
> 	--->	pred_wrapper(pred(io::di, io::uo) is det).
> 
> When you take the argument out of a pred_wrapper term, it will have
> the higher-order inst, so you can call it:
> 
>     Wrapper = pred_wrapper(P),
>     P(!IO)
> 
> This will save you having to specify higher-order insts everywhere.

Yes, I've done this before. It's needed because the standard libraries don't
provide for higher order insts. It's a workaround.

Cheers,
Volker

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