[Haskell-cafe] How to read a file and return a String?

[Richard A. O'Keefe]

The original poster wants to
 - read a file
 - get the contents as a String
 - break the string into lines
 - do something with the lines
 - and presumably print the result

Easy.  Put the following lines in a file called 'rf.hs':

file_name = "rf.hs"

main =
    readFile file_name >>= \string -> putStr (process (lines string))

process lines = maximum lines ++ "\n"

Then
    m% ghc rf.hs
    m% ./rf
=>  process lines = maximum lines ++ "\n"

Explanation:

   readFile file_name
	is an IO action which when performed will read the named file
	and deliver a string

   cmd1 >>= \x -> cmd2
	is an IO action which when performed will first perform cmd1,
	then bind its result to x, and then perform cmd2.
	>>= is the fundamental operation for chaining actions together.

   lines
	is a plain old String -> [String] function

   process
	is a plain old [String] -> String function, whatever you want

   putStr x
	is an IO action which when performed will write x

The run time environment causes main to be performed.