[ghc-steering-committee] Proposal #23: instance force

[Joachim Breitner]

Hi,

I’ll try to refrain from pouncing on every point raised, and I am not
trying to sway anyone’s mind (I don’t feel very strongly about the
proposal and will not shed a tear if it does not go through), but allow
me to comment on

Am Donnerstag, den 27.04.2017, 22:56 +0000 schrieb Simon Peyton Jones:
> No need to do this ill-specified "change the type of imported
> functions" stuff.

that I deliberately avoided talking about “Wanted” constraints and
other Outside-In terminology, which I consider implementation details
and which a language feature specification should, if at all possible,
avoid. I might not have succeeded in giving a good specification, but
there is a rather clear picture in my head, so I can clarify if there
are open questions.

> It'd work fine for
>         instance force C [a]
> too, which the proposal doesn't allow. 

Not quite true. If there is a

    instance C [a]

in scope around, then

    instance force C [a]

would work fine.


But you are right that the current wording (“from an empty context”)
would prohibit it if the instance

   instance D a => C [a]

where in scope.

Should the tides turn (which seems unlikely) I can improve this part.

Greetings,
Joachim




-- 
Joachim Breitner
  mail at joachim-breitner.de
  http://www.joachim-breitner.de/
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