[Haskell-beginners] Print values stored in an "Either" in the main method

[Mathew Phillips]

Ok now I'm even more confused! That works, but I thought that "let" was for
when the value inside the do block was pure, and the <- notation when it's
inside another monad?


On Thu, Feb 20, 2014 at 7:01 PM, David McBride <toad3k at gmail.com> wrote:

> 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 at 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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>
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