[m-users.] inst question

[Volker Wysk]

Am Donnerstag, dem 28.07.2022 um 13:51 +1000 schrieb Zoltan Somogyi:
> 2022-07-28 13:34 GMT+10:00 "M McDonough" <foolkingcrown at gmail.com>:
> > > What I don't understand, is why is the instantiatedness of LinkParts in line
> > > 35 "bound(link_parts(ground))" and not
> > "bound(link_parts(bound(left(ground)))" ?
> > 
> > You need to declare a mode which uses the inst you defined.
> 
> He did, because ...
> 
> > :- mode lpleft_in == lpleft >> lpleft.
> > :- pred fsf2(link_parts::lpleft_in, io.io::di, io.io::uo) is det.
> 
> ... the mode "in(lpleft)" in Volker's code expands to "lpleft >> lpleft".
> 
> > A pred that requires a certain input inst which works this
> > way can be annoying to use, as to go from an `in` link_parts in this
> > example you would need to write a disjunction and then destructure and
> > construct a new link_parts with an either of left, there isn't an
> > automatic way to do this without writing it out.
> 
> I think this "destructure and construct" is what Volker is missing.

Yes, that's it.

> The root cause of Volker's issue is the fact that the Mercury compiler
> does not track aliases (which has been the topic more than once
> on m-users). In this case, the code that Volker wrote
> 
>   LinkParts = link_parts(left(_)),
>   fsf3(LinkParts, !IO)
> 
> is seen by the compiler as
> 
>   LinkParts = link_parts(V1),
>   V1 = left(...),
>   fsf3(LinkParts, !IO)
> 
> (The compiler breaks down all unifications into primitive forms,
> none of which contain more than one function symbol, before doing
> any semantic checking on the program.)
> 
> After mode analysis processes V1 = left(...), the compiler obviously
> knows that V1 is bound to left/1. But, by then the compiler has
> already forgotten that V1 is an alias for the first argument of link_parts
> in LinkParts, so it does not know that the success of the V1 = left(...)
> unification affects the inst of LinkParts in any way. 

This makes it clear.

> However, if you
> construct a new version of LinkParts, called say LinkParts2, from V1,
> the compiler *will* know that LinkPart2's first arg is bound to left.

Yes, got it. My little test program works now. Thanks!


Volker
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