On Sun, Mar 25, 2007 at 01:28:53AM +0100, Marc A. Ziegert wrote:
hi!
i've just discovered this strange behaviour of existential quantifiers with runST:
--- Prelude Control.Monad.ST> :t runST (return ()) runST (return ()) :: ()
Prelude Control.Monad.ST> :t runST $ (return ()) <interactive>:1:9: Couldn't match expected type `forall s. ST s a' against inferred type `m ()' In the second argument of `($)', namely `(return ())'
Prelude Control.Monad.ST> ---
the same with "id runST undefined". is this a bug or an unsolved problem? i'm not sure wheather it is a part of the wanted feature of runST's type definition.
i did not find any discussions about this. can anyone enlighten me, please?
Neither. The type of id is: forall a. a -> a In Haskell, type variables range only over normal types, not rank-two types. Therefore the application (runST id) is illegal. (Sadly GHC is not able to produce a particularly good error message in this case.) (There is a GHC extension in the HEAD that allows this, but you are adviced not to rely on or even try to understand it.) Stefna