[Haskell-cafe] type checking that I can't figure out ....

[Daniel Fischer]

Am Mittwoch 03 Juni 2009 06:12:46 schrieb Michael Snoyman:

> I made two changes:
>
> 1. You had the arguments to M.lookup backwards.
> 2. lookup does not return any generalized Monad, just Maybe (I think that
> should be changed).

Data.Map.lookup used to return a value in any monad you wanted, I believe until 6.8 
inclusive. 
I don't think it's going to change again soon.

> I added the simple liftMaybe function to convert the
> Maybe result into something that will work with your state monad.
>
> Michael