After a little too long trying, I managed to get code from which the type system will infer that type, without using 'undefined':

*Main> :t map
map :: ((a -> a1) -> [a]) -> [a1]

import System.IO.Unsafe
import Data.IORef
import Prelude hiding (map)
import qualified Prelude as P

coerce a = unsafePerformIO (coerce' a)
    where
        ref = unsafePerformIO $ newIORef []
        coerce' a = do
                       writeIORef ref [a]
                       b <- readIORef ref
                       return (head b)

map' a2b f = P.map a2b (f a2b)
map = map' coerce

On 27/05/07, Stefan Holdermans <stefan@cs.uu.nl> wrote:

>>> map :: ((a -> b) -> [a]) -> [b]
>>
>> I am not following here: what do you mean? Clearly, this is not a
>> valid typing for map. Moreover, modulo undefinedness, there are no
>> functions with this typing.
>
> map _ = []

Ah, well, and that one, of course... :-)

Cheers,

Stefan
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