Control.Monad proposal: Add whenJust

Edward Kmett ekmett at
Wed May 15 18:14:32 CEST 2013

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.


On Wed, May 15, 2013 at 10:25 AM, Ben Millwood <haskell at>wrote:

> On Wed, May 15, 2013 at 08:01:52AM +0300, Roman Cheplyaka wrote:
>> * Ben Millwood <haskell at> [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.
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