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

When you do computations involving IO, you want to be in IO context. For instance, let's say we want to count the amount of rows in a file, and we're going to use readFile for this First, lets look at the types readFile :: FilePath -> IO String This is a computation with an IO result, which means we should only access it by IO actions. computeLines :: String -> Int computeLines = length . lines Lines is an example computation for calculating the amount of lines in a file. It's composed of the function lines :: String -> [String] which splits a string into a list of strings, divided by line break. This is the function we want to use the result of readFile on, but as it's IO, the type system won't let us. However, there's hope still countLines :: FilePath -> IO Int countLines fp = do file <- readFile fp return $ computeLines file By the powers of the do notation and IO actions we can compose the functions and still live in the comfortable context of IO actions. If you're not used to IO computations, this snippet will contain some concepts that probably are new to you: * The "do" notation; it's a syntactic sugar for doing sequential computations within a type, like IO actions. * The left pointing arrow (<-) is a type-safe way to extract a contextual value (like IO String) into a computable value (like String). * return packages a computation (like Int) into a contextual value (like IO Int). So what we do is fetching the result from readFile with <-, computes it (computeLines file) and package it from an Int to an IO Int. Best regards, Jonathan 2015-05-25 14:29 GMT+02:00 Magnus Therning :

On 25 May 2015 at 08:44, 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?

Start with reading some basic Haskell book/tutorial. That should tell you how to e.g. use 'do' notation, or `liftM`, to achieve what you want.

/M

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