I recently wrote
do
e <- (Left <$> m) <|> (Right <$> n)
more actions
case e of
Left x -> ...
Right y -> ...
This seems awkward. I know that protolude (and some other packages) define
-- Called eitherP in parser combinator libraries
eitherA :: Alternative f => f a -> f b -> f (Either a b)
But even with that, it all feels a bit manual. Is there some nice pattern I haven't seen for tightening it up?
I just noticed that OP expressed this same idea in a comment. I'm going to post my thoughts anyway.
Coyoneda is a neat trick, but it's a little overkill for this particular problem. I think all you need is regular old continuations.
Let's name those ...s:
do
e <- (Left <$> m) <|> (Right <$> n)
more actions
case e of
Left x -> fx x
Right y -> fy y
Then, we could instead have written this as:
do
e <- (fx <$> m) <|> (fy <$> n)
more actions
e
This is slightly subtle — it's important to use <$> there even though it looks like you might want to use =<< so that the result of the first line is actually a monadic action to be performed later rather than something that gets performed right away.