Hi David,
See Note [Error and friends have an "open-tyvar" forall] in MkCore. The
short answer is that error and undefined are treated magically by GHC:
the actual type of undefined is
forall (a :: OpenKind) . a
and both * and # are subkinds of OpenKind.
(There is a plan to get rid of this subkinding in favour of normal
polymorphism, but it hasn't been implemented yet. See
https://ghc.haskell.org/trac/ghc/wiki/NoSubKinds for more details.)
Hope this helps,
Adam
On 01/02/15 18:54, David Feuer wrote:
> If I define
>
> {-# LANGUAGE MagicHash #-}
>
> g :: Int# -> Int
> g 3# = 3
>
> myUndefined = undefined
>
> then this gives a sensible type error about a kind mismatch:
>
> usual :: Int
> usual = g myUndefined
>
> but this, oddly enough, compiles:
>
> peculiar :: Int
> peculiar = g undefined
>
> GHCi and the definition in GHC.Error agree that
>
> undefined :: a
>
> So why am I allowed to use it as a type of kind #?
--
Adam Gundry, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/
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