[Haskell-cafe] Data.Map lookup signature
ryani.spam at gmail.com
Tue May 12 16:12:38 EDT 2009
Maybe is an instance of Monad; the second signature is just more
general than the first.
class Monad m where
return :: a -> m a
(>>=) :: m a -> (a -> m b) -> m b
fail :: String -> m a
Map lookup only uses "return" and "fail"; for Maybe these are defined
return x = Just x
fail s = Nothing
So it's really the same thing.
On Mon, May 11, 2009 at 8:37 AM, Nico Rolle <nrolle at web.de> wrote:
> Hi everyone.
> The docs in the web on http://www.haskell.org/ghc/docs
> define Data.Map.lookup as follows:
> lookup :: Ord k => k -> Map k a -> Maybe a
> but my version of ghci does i like that:
> Data.Map.lookup :: (Ord k, Monad m) => k -> Data.Map.Map k a -> m a
> but i need the 1. one.
> my version of ghci is 6.8.2
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