How to compose writeFile with Either data type?

Question

I have a function that accepts some arguments and returns IO (Either String String), say

testEither :: Int -> IO (Either String String)
testEither 0 = return (Left "we got an error")
testEither _ = return (Right "everything is ok")

(Real function fetches some stuff from real world and might fail)

I want to send output from that function to writeFile fileName. Expected behavior: if I bind testEither 0 to writeFile "test.txt", I fail with Left ..., and if I call it with testEither 1, I get everything is ok in file test.txt.

I guess the type of the whole expression should be something like IO (Either String ()), but I may be wrong.

Answer 1

You can use the ErrorT¹ monad transformer to give you pure error handling on top of the IO monad:

import Control.Monad.Error

testEither :: Int -> IO (Either String String)
testEither 0 = return (Left "we got an error")
testEither _ = return (Right "everything is ok")

main = runErrorT $ do
    result <- ErrorT $ testEither 0
    lift $ writeFile "test.txt" result

_{¹ ErrorT appears to have been replaced with ExceptT in the newest version of mtl, but the functionality should be similar.}