Dananji,

Haskell explicitly separates "pure" from "impure", so "readFile <file>" returns not a string, but rather an action, which tells haskell to read a string. In order to "use" a result of some IO action as a pure value, you have several ways, most popula of which is a do-notation.

main :: IO ()

main = do

    s <- readFile "somefile.txt"

    putStrLn (show (doSplit s))

In the code above, variable "s" is "pure", it has type String and you can use it as you want. Do-notation is essentially a sugar to callback mechanism, where in order to use a value of some IO-computation you write a function-callback that will be called with a pure value of computation result. This code is the same as previous one, but without do-notation:

main :: IO ()
main =

    readFile "somefile.txt" >>= (\s ->

    putStrLn (show (doSplit s)))

I highly recommend reading "Learn You A Haskell" book http://learnyouahaskell.com/ , which explained these concepts really well to me.

On Mon, May 25, 2015 at 9:44 AM, Dananji Liyanage <dan9131@gmail.com> wrote:

Hi All,

I'm writing a code, where the input is read from a text file using:  

readValues = readFile "Input.txt"

Since the type of this is 'IO String', I can't use this in the consequent functions.

For an example: I want to split this as follows within another function

extractInput url method template

  | isURI url == True = getList values components

  | otherwise = []

  where components = splitTemplate readValues

        values = getURL (splitURL url) method

This gives the following error:

 Couldn't match type ‘IO String’ with ‘[Char]’

    Expected type: String

      Actual type: IO String

How can I solve this?

Thanks in advance! 

--

Regards,
Dananji Liyanage

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