[Haskell-cafe] Eta Reduction
haskell at nand.wakku.to
Thu Apr 3 00:40:30 UTC 2014
On Wed, 2 Apr 2014 14:25:55 -0700, John Lato <jwlato at gmail.com> wrote:
> On Apr 2, 2014 4:32 PM, "Niklas Haas" <haskell at nand.wakku.to> wrote:
> > On Wed, 2 Apr 2014 09:51:46 -0400, Edward Kmett <ekmett at gmail.com> wrote:
> > > We're not storing the instance as a slot in the constructor, so this
> > >
> > > 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
> > > 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?
> 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.
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