"Either Fmap" which Continues until a Right Result has been Reached

Question

If Either fmap is

(a -> b) -> p a a -> p a b

which stops mapping once a Left is returned.

What's name or type signature for a function which doesn't stop until it gets a Right result.

I suspect a bifunctor, but I really need it spelled out - don't quite get the subtlety of the logic of these things yet.

Perhaps some sort of fold fits also...

Have you tried asking [Hoogle](https://www.haskell.org/hoogle)? — Mark Seemann, Aug 25 '17 at 14:57
I have absolutely no idea what this question is asking. `Either fmap` means what? What is `p`? What's it mean for a function to "stop"? — dfeuer, Aug 26 '17 at 01:07

score 5 · Accepted Answer · answered Aug 25 '17 at 14:58

5

Data.Bifunctor.first :: (a -> b) -> Either a c -> Either b c

In ghci:

Data.Bifunctor> first (+1) (Left 0)
Left 1
Data.Bifunctor> first (+1) (Right 0)
Right 0

answered Aug 25 '17 at 14:58

Daniel Wagner

1 Answers1