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Cannot match expected type [a0] with actual type IO ()

What is wrong in my code:

insertValue file x = if x == "10" then "ok" else do putStrLn "Error"; file
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3 answers

In an if..then..else expression if..then..else both branches must be of the same type.

One branch:

 "10" :: String

Another branch:

 do putStrLn "Error"; file :: IO ??

Since I'm not sure what you are trying to do (and the compiler is not sure either), I do not know how to fix the code.

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You need to use return :: a -> IO a to "raise" your strings in an IO String :

 insertValue file x = if x == "10" then return "ok" else do putStrLn "Error" return file

But are you sure you don't want to call putStrLn "ok" (instead of return "ok" ) and return Maybe? Otherwise, you return file or "ok" and your caller cannot determine if there was an error calling insertValue in a file named "ok".

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ok is of type String, while an else clause is of type IO (). In Haskell, "if" is an expression, so they must match.

Its hard to help anymore without knowing what you are trying to do.

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