Re: [Haskell-beginners] Using IO values for computations

Thank you for the nice explanation! I understood it for some extent, and will read further on I/O. On Mon, May 25, 2015 at 9:53 PM, Kostiantyn Rybnikov wrote:

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 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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-- Regards, Dananji Liyanage