How to read a file and return a String?
I want to read a text file, and store it in a *String*. But readFile will get *IO String*. I search with google and they tell me it is not necessarily to do so. Can you explain to me why is this? Furthermore, How to read a file and store it in a String? In fact, I want to read a file and split it into *[String]* with *lines*function. Here is what I have tried. linkhttps://github.com/eccstartup/findDict/blob/master/src/Data/Dict/Utils.hsI think I could directly operate on *[String]*. But it is not true for me. [1] http://www.haskell.org/hoogle/?hoogle=IO+String+-%3E+String Yi Lu
On Wed, Sep 4, 2013 at 10:21 AM, yi lu
I want to read a text file, and store it in a *String*. But readFile will get *IO String*. I search with google and they tell me it is not necessarily to do so. Can you explain to me why is this? Furthermore, How to read a file and store it in a String?
You do not do so directly. An IO action is a promise to produce a value, not an actual value. (readFile contains a String in the same way the "ls" or "dir" command contains a list of files.) I suggest you take a look at http://learnyouahaskell.com/input-and-output#files-and-streams to see how IO works in Haskell. tl;dr: use do notation (which lets you pretend to a limited extent that you can see the String in an IO String) or >>= or fmap to attach a callback to the IO "promise". readFile >>= (something that operates on a String and produces an IO whatever) do s <- readFile (something that operates on a String and produces an IO whatever) Note that in the end it's still in IO. You can't escape it. (There are actually ways to "escape" but they will get you into trouble fairly quickly because they don't work the way you want them to.) -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
On Wed, Sep 04, 2013 at 10:21:37PM +0800, yi lu wrote:
I want to read a text file, and store it in a *String*. But readFile will get *IO String*. I search with google and they tell me it is not necessarily to do so. Can you explain to me why is this? Furthermore, How to read a file and store it in a String?
You need to lift 'lines' into the IO functor, rather than trying to "remove" the String from IO (which doesn't make sense). Try fmap lines (readFile "filename") Tom
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.
participants (4)
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Brandon Allbery -
Richard A. O'Keefe -
Tom Ellis -
yi lu