Re: Map.lookup in 6.4.1

Hi Serge,

Prelude> Data.Map.singleton 'a' 'b' {'a':='b'}

Prelude> Data.Map.lookup 'a' $ Data.Map.singleton 'a' 'b'

Prelude> -------------------------------

Must not it print Just 'b' instead of printing emptyness? Further:

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

*** Exception: user error (Data.Map.lookup: Key not found) ----------

Instead, must not it print Nothing ?

'Data.Map.lookup' has the return type 'm a', that is returns the value within an arbitrary monad of type m, which need not even be a member of the class 'Show'. You need to further specify the type of the monad. '(Data.Map.lookup 'a' $ Data.Map.singleton 'a' 'b')::Maybe Char' does what you expect. Regards, Jens