[Haskell-cafe] Data.Map lookup signature

[Ryan Ingram]

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
as follows:
   return x = Just x
   fail s = Nothing

So it's really the same thing.

  -- ryan

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:
> http://www.haskell.org/ghc/docs
> 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
> regards
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