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