Yes it does. Thanks. For the sake of consistency, I'd rather even have separate functions with funny-looking types than hidden magic. That is, we could hypothetically have undefined# :: forall (a :: #) . a error# :: forall (a :: #) . String -> a There's no mechanism in Haskell to create things with such types, but at least that would make the strange types explicit. Of course, if someone can do something better, well, better is better. On Sun, Feb 1, 2015 at 2:07 PM, Adam Gundry wrote:

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/