Haskell read lines of file

Question

I want to read a whole file into a string and then use the function lines to get the lines of the string. I'm trying to do it with these lines of code:

main = do
   args <- getArgs
   content <- readFile (args !! 0)
   linesOfFiles <- lines content

But I'm getting the following error by compiling ad it fails:

Couldn't match expected type `IO t0' with actual type `[String]'
In the return type of a call of `lines'
In a stmt of a 'do' block: linesOfFiles <- lines content

I thought by binding the result of readFile to content it will be a String DataType, why isn't it?

Daniel Fischer · Accepted Answer · 2012-06-13 20:04:23Z

55

I thought by binding the result of readFile to content it will be a String DataType, why isn't it?

It is a String indeed, that's not what the compiler complains about. Let's look at the code:

main = do
   args <- getArgs
   content <- readFile (args !! 0)

Now content is, as desired, a plain String. And then lines content is a [String]. But you're using the monadic binding in the next line

   linesOfFiles <- lines content

in an IO () do-block. So the compiler expects an expression of type IO something on the right hand side of the <-, but it finds a [String].

Since the computation lines content doesn't involve any IO, you should bind its result with a let binding instead of the monadic binding,

   let linesOfFiles = lines content

is the line you need there.

answered Jun 13, 2012 at 20:04

Daniel Fischer

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3 Comments

Gabriella Gonzalez Over a year ago

Additionally, compare this to the type of getArgs and readFile, for completeness.

kjh Over a year ago

How does this solution work? You can't pass content to lines because the type of lines is String -> [String] and the type of content is IO String

Daniel Fischer Over a year ago

@kjh No, content is a String. The type of readFile (args !! 0) is IO String, and we bind content to the "result" of that IO-action. The construct do { a <- action; stuff; } desugars into action >>= \a -> stuff, if action has type IO t, then a has type t.

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