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

Bertram Felgenhauer bertram.felgenhauer at googlemail.com
Wed Jun 3 20:13:59 EDT 2009

Michael Snoyman wrote:
> On Wed, Jun 3, 2009 at 8:42 AM, Daniel Fischer <daniel.is.fischer at web.de>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:

The discussion about the proposal can be found here:

(There's even the suggestion of adding a function like liftMaybe to
Data.Maybe, but apparently nobody turned that into a formal proposal.)



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