[m-users.] IO argument clobbering

[Sean Charles (emacstheviking)]

Finally, you made my day Zoltan!

:)



> On 17 Jul 2023, at 16:34, Zoltan Somogyi <zoltan.somogyi at runbox.com> wrote:
> 
> 
> On 2023-07-17 17:26 +02:00 CEST, "Sean Charles (emacstheviking)" <objitsu at gmail.com <mailto:objitsu at gmail.com>> wrote:
>> [A]  :- type callback == (pred(world, world, io, io)).
>> [B]  :- inst callback == (pred(in, out, di, uo) is det).
>> 
>> [C]  :- pred run_loop(
>>      callback::in(callback), world::in, world::out, io::di, io::uo
>>  ) is det.
>> 
>> In an attempt to demystify it all then, at point [A] I am defining an equivalence type which is merely a Mercury way of defining something similar to a C preprocessor macro in that the use of callback *as a type* specified will just cause the compiler to use the value on the right hand side of '=='. That much I understand, I think!
>> 
>> At [B], I am defining an instantiatedness tree, that although it has the same name as that used at [A], bears no actual relationship to it in any way.
>> 
>> At [C], I am declaring that run_loop/5 is a predicate that takes an argument of *type* callback, and that that it is an input, and that the input MUST have the instantiatedness defined by the definition at [B], the first use of callback being the type, and the in(callback) is stating that the value MUST be ground i.e. contain no free variables.
>> 
>> Is that not even wrong, or close?
> 
> What you write above is exactly right.
> 
> Note that giving a type and an inst the same name is just a naming
> convention. It is useful in that it tells people who are familiar with
> the convention that the type and the inst are expected to be used together.
> However, it does have the drawback that it can, and often will, confuse
> people who are not familiar with the convention, and who don't know
> that when the name (such as "callback" in this case) is used in a context
> where the compiler expects a type, it will be interpreted as the name of
> the type, whereas in contexts where the compiler expects an inst,
> it will be interpreted as the name of the inst.
> 
> Zoltan.

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