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

[Simon Peyton Jones]

|  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

I agree with that, and it's a valid criticism of my language.  However, if you want to reason about what type should be inferred for

f x = x==x && x>x

(which is a user-facing thing, totally part of the language spec), then it's probably easier to say

"From the use of (==) you get a wanted (Eq a) constraint, where x::a.  From the use of (>) you get a wanted (Ord a).  Both can be derived from a Given (Ord a)."  or language like that.  Understanding typing rules is, I think, harder.

Moreover, it's quite hard to explain fundeps using typing rules.  (Try it!)   I was trying to draw the analogy, since we already have fundeps.

Simon


|  -----Original Message-----
|  From: ghc-steering-committee [mailto:ghc-steering-committee-
|  bounces at haskell.org] On Behalf Of Joachim Breitner
|  Sent: 28 April 2017 01:20
|  To: ghc-steering-committee at haskell.org
|  Subject: Re: [ghc-steering-committee] Proposal #23: instance force
|  
|  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
|  
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