I'm a haskell beginner and got stuck with IO action behavior.
main() :: IO() retval = if something then "foo" else `return value of IO String action` i.e getLine print retval How can I make this code work??
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2 Answers
Both branches of if have to have the same type. Since "foo" is String, the else part has to be a String too (but not an IO String).
You can do
retval = if something then return "foo" else getLine in order to make all parts of the if of type IO String. Here, return converts the string "foo" to an IO action which just returns "foo".
Then, you need to extract the String back out of the IO String because print expects a string (i.e. you carry out the IO action):
retval <- if something then return "foo" else getLine 
1 Comment
Sho
Thanks for kind reply :) I understand now
Both the then and else value in an if need to have the same type. You need to convert String to IO String.
The return function will do this for you.
main:: IO() main = do retval <- if something then return "foo" else getLine print retval Note that because the this is of type IO a, you also need to assign the value of retVal using "<-", not let .. = ...
