[Haskell-beginners] Using IO values for computations

[Kostiantyn Rybnikov]

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 at 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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