CODEWITHSUNDEEP
haskellmonads
Sho
Sho

Reputation: 25

Dealing with IO action

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??

Upvotes: 2

Views: 146

Answers (2)

undur_gongor
undur_gongor

Reputation: 15954

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

Upvotes: 2

jamshidh
jamshidh

Reputation: 12060

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 .. = ...

Upvotes: 1

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