Because you didn't put all your code, I'm not sure if this is correct, but my best guess is that you expect runEval to return Either String a, but the way you are using it I suspect you actually want its type to be IO (Either String a).

Alternatively change

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

to

let result = runEval env (eval (Plus (Var "x") (Lit 2)))

On Thu, Feb 20, 2014 at 7:19 PM, Mathew Phillips <mathewrphillips@gmail.com> wrote:

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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