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

Michael Snoyman wrote:

On Wed, Jun 3, 2009 at 8:42 AM, Daniel Fischer wrote:

...

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

...

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.

Is there a reason why it only returns in the Maybe monad? I often times have to write a liftMaybe function to deal with that.

Here's the proposal that changed it: http://hackage.haskell.org/trac/ghc/ticket/2309 The discussion about the proposal can be found here: http://www.haskell.org/pipermail/libraries/2008-May/009698.html (There's even the suggestion of adding a function like liftMaybe to Data.Maybe, but apparently nobody turned that into a formal proposal.) Regards, Bertram