On Mon, Oct 5, 2009 at 9:48 AM, Maria Boghiu
Hey.
I'm trying to configure Xmonad, the window manager.
for this purpose, I'm trying to read the workspaces variable in the code below from file. As you can see, I do workspaces = readWS where
readWS = do l <- readFile "~/bla" return l
I get an error saying I am mismatching types IO [String] and [String]. But I thought that once one does l <- readFile "~/bla", l is the a string or, in this case, a String list. If I run that line in prelude it seems to be working fine, prints the content of the file "~/bla" on screen (though I'm not sure if as a String or as a list of Strings, i.e. [String])
Can anyone help please? I've been struggling with this for days...
94 main = do 95 xmonad 96 $ defaultConfig { 97 manageHook = manageDocks <+> myManageHook <+> manageHook defaultConfig, 98 layoutHook = avoidStruts $ layoutHook defaultConfig, 99 logHook = dynamicLogWithPP $ conkyPP "", 100 terminal = "gnome-terminal", 101 keys = myKeys, 102 workspaces = readWS, 103 mouseBindings = myMouseBindings, 104 focusedBorderColor = "#008E00", 105 borderWidth = 3 106 --modMask = mod1Mask, --This is to rebind mod key 107 } `additionalKeys`
A quick tip: when GHC gives you a compile error like that, it's because you've made an error where 'what I say' is not 'what I [want to] do'. In this case, you're *saying* 'use readFile, which returns a String', and *doing* 'ok, here, use this [String]'. The way to know that it's an issue is to find out what type readFile is; in ghci you can do ':type readFile' and it'll tell you String -> IO String. Now you know workspaces needs [String], so you need something which goes 'String -> [String]'. If you don't already know that that is 'lines', then you can ask Hoogle http://www.haskell.org/hoogle/ that question: http://www.haskell.org/hoogle/?hoogle=String+-%3E+[String] and it will give you as the first hit exactly what you need. (Hoogle is great for beginners who don't know the libraries.) Obviously, this is a real issue; there's no way the compiler could possibly figure out what the right thing to do because there are so many ways to do String->[String] (just consider the specific example of 'lines' - how does it handle Windows v Unix line-endings?). So you have to tell it. The monad issue with the difference between IO [String] and [String] I will leave to others to explain. :) -- gwern