Add expandEnv function and use it in PipeReader family of monitors

expandEnv takes a string and expands the environment variables it can
find. variable substringing (e.g. ${VAR:1} to lop off the first
character) is not supported, but $VAR and ${VAR} formats are, with the
former being delimited by punctuation, but not underscores.

1 files changed, 48 insertions, 0 deletions

diff --git a/src/Environment.hs b/src/Environment.hs
new file mode 100644
index 0000000..ebdf733
--- /dev/null
+++ b/src/Environment.hs
@@ -0,0 +1,48 @@
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  XMobar.Environment
+-- Copyright   :  (c) William Song
+-- License     :  BSD-style (see LICENSE)
+--
+-- Maintainer  :  Will Song <incertia@incertia.net>
+-- Stability   :  stable
+-- Portability :  portable
+--
+-- A function to expand environment variables in strings
+--
+-----------------------------------------------------------------------------
+module Environment where
+
+import Data.Maybe           (fromMaybe)
+import System.Environment   (lookupEnv)
+
+expandEnv :: String -> IO String
+expandEnv "" = return ""
+expandEnv (c:s) = case c of
+  '$'       -> do
+    envVar <- fromMaybe "" <$> lookupEnv e
+    remainder <- expandEnv s'
+    return $ envVar ++ remainder
+    where (e, s') = getVar s
+          getVar "" = ("", "")
+          getVar ('{':s) = (takeUntil "}" s, drop 1 . dropUntil "}" $ s)
+          getVar s = (takeUntil filterstr s, dropUntil filterstr s)
+          filterstr = ",./? \t;:\"'~`!@#$%^&*()<>-+=\\|"
+          takeUntil f = takeWhile (not . flip elem f)
+          dropUntil f = dropWhile (not . flip elem f)
+
+  '\\' -> case s == "" of
+    True  -> return "\\"
+    False -> do
+      remainder <- expandEnv $ drop 1 s
+      return $ escString s ++ remainder
+      where escString s = let (cc:ss) = s in
+              case cc of
+                't' -> "\t"
+                'n' -> "\n"
+                '$' -> "$"
+                _   -> [cc]
+
+  _    -> do
+    remainder <- expandEnv s
+    return $ c : remainder