Re: Control.Monad proposal: Add whenJust

16 May 2013

      +1.

Currently something is broken since in the presence of

   import Data.Traversable (mapM)

the rest of the import list looks like this:

   import Prelude hiding (mapM)
   import Control.Monad.Identity hiding (mapM)
   import Control.Monad.State hiding (mapM)
   import Control.Monad.Error hiding (mapM)
   import Control.Monad.Reader hiding (mapM)

I don't know why all these modules export a mapM that only works for 
lists, rather than the generic mapM from Traversable.

--Andreas

On 16.05.13 9:22 AM, Ganesh Sittampalam wrote:
...
-1 for the reasons given by Henning.
Also more polymorphic types generally cause worse error messages.
On 15/05/2013 17:14, Edward Kmett wrote:
...
Personally, I'd be all for just moving Foldable (and Traversable) into
the Prelude and retiring the monomorphic versions of the functions they
supply. Both abstractions have born the test of time, and its hard to
even envision Haskell without them at this point.
I'm somewhat leery that we coud get this proposal past the "but it makes
it harder to introduce people to Haskell" backlash, but I'd
wholeheartedly support it.
-Edward
On Wed, May 15, 2013 at 10:25 AM, Ben Millwood mailto:haskell@benmachine.co.uk> wrote:
On Wed, May 15, 2013 at 08:01:52AM +0300, Roman Cheplyaka wrote:
* Ben Millwood mailto:haskell@benmachine.co.uk> [2013-05-12 10:11:01+0100]
You can mostly minimise harm by only hiding specific things, but
             that's still more effort than I feel like I should have to
             go to. I
             think if we decide that the Foldable approach is useful
             enough to go
             in base, we should not make it a second-class citizen.
... except it is in base already :)
Roman
Yeah, sorry, to clarify: *since* we think it is important enough to
     go in base, we should make it easy to use as well.
-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Theoretical Computer Science, University of Munich
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andreas.abel@ifi.lmu.de
http://www2.tcs.ifi.lmu.de/~abel/