CODEWITHSUNDEEP
haskellfunctional-programming
jchen114
jchen114

Reputation: 25

Haskell Expected type: IO Char Actual type: String

Here's my code: 

    mingle :: String -> String -> String
    mingle (a:as) (b:bs) = ([a] ++ [b]) ++ mingle as bs
    mingle [] [] = []
    main = putStrLn "Enter 1st String:"
                >> getLine
                >>= \a -> read a >> putStrLn "Enter 2nd String:"
                >> getLine
                >>= \b -> read b >>= mingle a b

Error: 

MingleStrings.hs:10:45:
    Couldn't match type ‘[]’ with ‘IO’
    Expected type: IO Char
      Actual type: String
    In the second argument of ‘(>>)’, namely ‘mingle a b’
    In the expression: read b >> mingle a b

I was under the impression that read would be able to turn an IO type to a standard haskell type. None of the other posts about dealing with IO seemed to help.

Upvotes: 0

Views: 1481

Answers (1)

Sibi
Sibi

Reputation: 48654

read doesn't return IO type as evidenced by it's type signature:

λ> :t read
read :: Read a => String -> a

What you want to do is this:

main = putStrLn "Enter 1st String:"
       >> getLine
       >>= \a -> putStrLn "Enter 2nd String:"
       >> getLine
       >>= \b -> return $ mingle a b

Since mingle is a pure function, you have to use return to inject IO on top of that. Also note that your mingle function doesn't handle all the cases. So you may want to fix that.

Upvotes: 3

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