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/