From 426c931d5b0ebc6d53396c34ec38eb342be501c3 Mon Sep 17 00:00:00 2001
From: jao <jao@gnu.org>
Date: Sun, 25 Nov 2018 07:35:54 +0000
Subject: Refactoring: Xmobar.Run

---
 src/lib/Xmobar/Run/Runnable.hs | 60 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 src/lib/Xmobar/Run/Runnable.hs

(limited to 'src/lib/Xmobar/Run/Runnable.hs')

diff --git a/src/lib/Xmobar/Run/Runnable.hs b/src/lib/Xmobar/Run/Runnable.hs
new file mode 100644
index 0000000..962166e
--- /dev/null
+++ b/src/lib/Xmobar/Run/Runnable.hs
@@ -0,0 +1,60 @@
+{-# 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 Xmobar.Run.Runnable where
+
+import Control.Monad
+import Text.Read
+import Xmobar.Run.Types (runnableTypes)
+import Xmobar.Run.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
+     trigger (Run a) = trigger 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 (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
-- 
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