[Haskell-cafe] Eta Reduction

[Dan Doel]

On Wed, Apr 2, 2014 at 8:40 PM, Niklas Haas <haskell at nand.wakku.to> wrote:

> 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.
>

This is pretty weird, but here's what I think is going on:

F requires an argument of type forall x. Show x => x -> String. This
requires abstracting over a dictionary for Show x. So at the core level,
this gets expanded to something like:

   \showd -> undefined

which is non-bottom.

Even if you annotate undefined with the above type, You'll probably end up
with:

    (\showd -> undefined) defaultd `seq` ()

after GHC reinstantiates the polymorphic type and then defaults, which will
be undefined. So you can only observe this by wrapping the polymorphic
expression in a newtype, to keep it abstracted.

I don't know that this qualifies as a bug, but it's definitely pretty
subtle behavior.

-- Dan
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