Haskell non-exhaustive patterns with matching Either

Vote count:

0

I wish to implement a function eitherMap with following spec:

eitherMap :: (a -> c) -> (b -> d) -> Either a b -> Either c d

which takes in two functions and map one of the function to input.

My current implementation is straight-forward as:

eitherMap fa fb (Left a) = Left (fa a)
eithermap fa fb (Right b) = Right (fb b)

It works fine with some cases but not some others involving list function.

> eitherMap (+2) reverse (Left 3)
Left 5
> eitherMap (+2) reverse (Right [2,3,4])
*** Exception: ...: Non-exhaustive patterns in function eitherMap

Can somebody give me a hint why this is happening? Thanks!

asked 1 min ago

Ray

1,557

Haskell non-exhaustive patterns with matching Either