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
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/