Re: Proposal: add Control.Monad.whenJust ∷ (Monad m) ⇒ Maybe α→(α→ m ()) → m ()

Henning Thielemann lemming at
Thu Dec 1 10:44:50 CET 2011

On Thu, 1 Dec 2011, Mikhail Vorozhtsov wrote:

> On 11/30/2011 11:26 PM, Evan Laforge wrote:
>> On Tue, Nov 29, 2011 at 11:11 PM, Liyang HU< at>  wrote:
>>> This particular pattern crops up a lot for me. Does it for anyone else?
>> Yep, I have that in my personal library, and use it fairly often.
>> I prefer it to Foldable.forM_ because it's explicit about the type it
>> takes, forM_ sounds too much like a loop.
> Well, it is a loop over collection, which in the case of Maybe can contain at 
> most one element.

Hoogle even lists forM_ and for_ in the second and third position when 
searching for the signature
    "Maybe a -> (a-> m ()) -> m ()"

Nonetheless I also needed whenJust recently and missed "for_"!

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