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

It seems like if we could get fail out of Monad and into something like MonadFail/Zero, then it might make sense to make a lookup that returned an instance of that instead? Dan On Wed, Jun 3, 2009 at 8:13 PM, Bertram Felgenhauer wrote:

Michael Snoyman wrote:

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

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

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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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe