I have a module that looks like this

runEval :: (Show a) => Env -> Eval a -> Either String a

runEval = ...

data Value = IntVal Integer

main :: IO ()

main = do

    let env = Map.fromList [("x",IntVal 3)]

    result <- runEval env (eval (Plus (Var "x") (Lit 2)))

    case result of

        Left err -> putStrLn "Error: "++err

        Right (IntVal i) -> print i

when I call runEval from ghci I get back a result just fine 

let env = Map.fromList [("x",IntVal 3)]

runEval env (eval (Plus (Var "x") (Lit 2))) 

This outputs "Right (IntVal 5)" as I would expect. But when I try and compile my main method I get the following error.

Couldn't match expected type `Value'

with actual type `Either t1 Value'

    In the pattern: Right (IntVal i)

    In a case alternative: Right (IntVal i) -> print i

    In a stmt of a 'do' block:

      case result of {

        Left err -> print err

        Right (IntVal i) -> print i }

I also tried using (putStrLn . either show show) result, but this gave me the following error. 

Couldn't match expected type `Either a0 b0'

with actual type `Value'

    In the first argument of `putStrLn . either show show', namely

      `result'

    In a stmt of a 'do' block: (putStrLn . either show show) result

    In the expression:

      do { let env = Map.fromList ...;

           result <- runEval3 env (eval3 (Plus (Var "x") (Lit 2)));

           (putStrLn . either show show) result }

Why is it when I try and pattern match on Either String Value it says result is of type "Value", but when I try and use either it says result is of type "Either String Value"?

Matt P.

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