Re: [Haskell-cafe] Eta Reduction

2 Apr 2014

      On Wed, 2 Apr 2014 14:25:55 -0700, John Lato  wrote:
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On Apr 2, 2014 4:32 PM, "Niklas Haas"  wrote:
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On Wed, 2 Apr 2014 09:51:46 -0400, Edward Kmett  wrote:
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We're not storing the instance as a slot in the constructor, so this
isn't.
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data Foo a where
   Foo :: Seq a => Int -> a -> Foo a
or equivalently
data Foo a = Seq a => Foo !Int !a
Why not? Say we define Seq to something like
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type family Seq (a :: *) :: Constraint where
  Seq (a -> b) = 1 ~ 0
  Seq t = ()
What would be the drawback in this scenario?
I think that would be acceptable if we posit the existence of a valid Seq a
dictionary.  I think that would be possible for all builtin types and
anything defined via data.  But what about e.g.
newtype F = F (forall x. Show x => x-> String)
Oh, fair point; this particular F apparently doesn't break it but if you
remove the Show x constraint, it does.

Actually, is that a bug in GHC?
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λ newtype F = F (forall x. Show x => x -> String)
λ F undefined `seq` ()
()
λ undefined `seq` ()
*** Exception: Prelude.undefined
I'm not sure how to interpret this output.