What is the story behind the type of undefined?

[Adam Gundry]

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/