Map.lookup in 6.4.1 (and 6.4.2)

[Christian Maeder]

Serge D. Mechveliani schrieb:
> Please, what is the matter with  Map.lookup ?

Map.lookup results in any monad. At the ghci prompt that is the IO monad
(and not Maybe).

Prelude> :t Data.Map.lookup
Data.Map.lookup :: forall a (m :: * -> *) k.
                   (Ord k, Monad m) =>
                   k -> Data.Map.Map k a -> m a

Prelude> x <- Data.Map.lookup 'a' $ Data.Map.singleton 'a' 'b'

<interactive>:1:0: Warning: Defined but not used: `x'
Prelude> x
'b'

(N.B. this warning is confusing, but I've set -Wall)

Supply a type signature if you want a different result:

Prelude> Data.Map.lookup 'a' $ Data.Map.singleton 'a' 'b' :: Maybe Char
Just 'b'

HTH Christian