Search code examples
haskellmonadsmonad-transformers

How do you lift a binary function to monad transformers?

I know you can lift functions to monads with liftM, but I it doesn't seem to work with binary functions.

I tried to lift (+) but it didn't work

a = return 1 :: ExceptT String Maybe Int
b = return 2 :: ExceptT String Maybe Int
liftM (+) a b

• Couldn't match expected type ‘ExceptT String Maybe Int -> t’
                  with actual type ‘ExceptT String Maybe (Int -> Int)’
Solution

  • You can use one of these:

    liftM2 (+) a b
liftA2 (+) a b
(+) <$> a <*> b