How to convert Haskell System.Directory getHomeDirectory to a regular String?

Question

I'm a Haskell noob and, at the moment, only use it to configure xmonad.

I want to put my config into a git repo, for that I want to not have to hardcode my home dir to grab my icons.

I checked out http://www.haskell.org/haskellwiki/How_to_get_rid_of_IO but I am just too ignorant to understand it.

hd h = h =<< getHomeDirectory

getIcon::String -> String
getIcon out = ( "^i("++hd++".xmonad/dzen2/"++out )

Is this actually possible? If so, how? I don't want to operate on the directory, I just want the path, as a String and it's killing me.

Error is:

Couldn't match expected type `[Char]'
            with actual type `(FilePath -> IO b0) -> IO b0'
In the first argument of `(++)', namely `hd'
In the second argument of `(++)', namely
  `hd ++ ".xmonad/dzen2/" ++ out'
In the expression: ("^i(" ++ hd ++ ".xmonad/dzen2/" ++ out)

Looks to me like the IO monad was not removed at all.

Update: Alright. I'll learn how to adapt to IOs rules, until then I'll keep things hardcoded and clone the config file with a script that will replace the appropriate bits.

Answer 1

Your getIcon has the wrong type, since getHomeDirectory does IO:

getIcon :: String -> IO String
getIcon out = do
     hd <- getHomeDirectory
     return $ "^i(" ++ hd ++ ".xmonad/dzen2/" ++ out

Remember that Haskell distinguishes code that has side effects -- such as reading your hard disk -- via the type IO.

So the caller would be in IO too:

main = do
    s <- getIcon "foo"
    .. now you have a regular string 's' ...

Answer 2

Can you change the code at the point where you're calling getIcon?

If you can get the home directory before calling it, you could do

getIcon :: String -> String -> String
getIcon out hd = ( "^i("++hd++".xmonad/dzen2/"++out )

and then wherever you're calling from (assuming it is in IO as well)

someIOFunction = do
    things
    ....
    hd <- getHomeDirectory
    getIcon out hd

Just to point out a last resort if nothing else works, there is unsafePerformIO, but I've never actually had to use it (and I feel like it is generally frowned upon), so I can't help you too much there.

Answer 3

You can "break out" of other monads, but you can't break out of the IO monad. Something like this in your Xmonad configuration file is probably what you want:

getIcon::String -> String
getIcon out = ( "^i("++hd++".xmonad/dzen2/"++out )

main =
   h <- getHomeDirectory
   let myIcon = getIcon "Firefox"
   xmonad $ desktopConfig . . . -- use myIcon somewhere in this expression

Answer 4

It's possible to unwrap certain IO values at compile-time, using Template Haskell. One's home directory is a reasonable candidate, since it doesn't change particularly often. A Template Haskell solution for getting the home dir as a string at compile-time might be coded like this:

{-# LANGUAGE TemplateHaskell #-}
module HomeDirectory where

import Language.Haskell.TH
import System.Directory

myHome :: String
myHome = $(LitE . StringL `fmap` runIO getHomeDirectory)

Be warned that Template Haskell is a notoriously tricky beast, though. This example is simple enough, but it can easily end up complex and confusing.