don't understand this liftM2 behaviour in Fay

Question

I have this haskell code which behaves as expected:

import Control.Monad

getVal1 :: Maybe String
getVal1 = Just "hello"

getVal2 :: Maybe String
getVal2 = Just "World"

main = process >>= putStrLn

process :: IO String
process = case liftM2 operation getVal1 getVal2 of
    Nothing -> error "can't run operation, one of the params is Nothing"
    Just result -> result

operation :: String -> String -> IO String
operation a b = return $ a ++ b

However when transposed to Fay, it doesn't typecheck:

{-# LANGUAGE NoImplicitPrelude, EmptyDataDecls #-}

import Prelude
import FFI

liftM2 f m1 m2 = do { x1 <- m1; x2 <- m2; return (f x1 x2) }

getVal1 :: Maybe String
getVal1 = Just "hello"

getVal2 :: Maybe String
getVal2 = Just "World"

main = process >>= putStrLn

process :: Fay String
process = case liftM2 operation getVal1 getVal2 of
    Nothing -> error "can't run operation, one of the params is Nothing"
    Just result -> result

operation :: String -> String -> Fay String
operation a b = return $ a ++ b

The compile error is:

fay: ghc: 
TestFay.hs:17:33:
    Couldn't match expected type `Fay String'
                with actual type `Maybe String'
    In the second argument of `liftM2', namely `getVal1'
    In the expression: liftM2 operation getVal1 getVal2
    In the expression:
      case liftM2 operation getVal1 getVal2 of {
        Nothing
          -> error "can't run operation, one of the params is Nothing"

I'm not exactly following the problem here. Actually I even tried to remove the import for Control.Monad in the GHC code and paste the liftM2 as in the Fay code, but it still typechecks properly... Any option of using such functions such as liftMx in Fay, or am I missing something completely here?

This is Fay 0.16.0.3... Maybe I should try upgrading to 0.17?

Answer 1

I suspect that do notation in Fay works for the Fay monad only, because AFAIK Fay does not support type classes. Looking at the Fay Prelude, I see that (>>=) and return are monomorphic, specialized to the Fay monad.