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+{-# LANGUAGE ExistentialQuantification, MultiParamTypeClasses, FlexibleContexts, FlexibleInstances #-}
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  Xmobar.Runnable
+-- Copyright   :  (c) Andrea Rossato
+-- License     :  BSD-style (see LICENSE)
+--
+-- Maintainer  :  Jose A. Ortega Ruiz <jao@gnu.org>
+-- Stability   :  unstable
+-- Portability :  unportable
+--
+-- The existential type to store the list of commands to be executed.
+-- I must thank Claus Reinke for the help in understanding the mysteries of
+-- reading existential types. The Read instance of Runnable must be credited to
+-- him.
+--
+-- See here:
+-- http:\/\/www.haskell.org\/pipermail\/haskell-cafe\/2007-July\/028227.html
+--
+-----------------------------------------------------------------------------
+
+module Runnable where
+
+import Control.Monad
+import Text.Read
+import Config (runnableTypes)
+import Commands
+
+data Runnable = forall r . (Exec r, Read r, Show r) => Run r
+
+instance Exec Runnable where
+     start (Run a) = start a
+     alias (Run a) = alias a
+
+instance Show Runnable where
+    show (Run x) = show x
+
+instance Read Runnable where
+    readPrec = readRunnable
+
+class ReadAsAnyOf ts ex where
+    -- | Reads an existential type as any of hidden types ts
+    readAsAnyOf :: ts -> ReadPrec ex
+
+instance ReadAsAnyOf () ex where
+    readAsAnyOf ~() = mzero
+
+instance (Show t, Read t, Exec t, ReadAsAnyOf ts Runnable) => ReadAsAnyOf (t,ts) Runnable where
+    readAsAnyOf ~(t,ts) = r t `mplus` readAsAnyOf ts
+              where r ty = do { m <- readPrec; return (Run (m `asTypeOf` ty)) }
+
+-- | The 'Prelude.Read' parser for the 'Runnable' existential type. It
+-- needs an 'Prelude.undefined' with a type signature containing the
+-- list of all possible types hidden within 'Runnable'. See 'Config.runnableTypes'.
+-- Each hidden type must have a 'Prelude.Read' instance.
+readRunnable :: ReadPrec Runnable
+readRunnable = prec 10 $ do
+                 Ident "Run" <- lexP
+                 parens $ readAsAnyOf runnableTypes