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|
{-# LANGUAGE ExistentialQuantification, MultiParamTypeClasses, FlexibleContexts, FlexibleInstances #-}
-----------------------------------------------------------------------------
-- |
-- Module : Xmobar.Runnable
-- Copyright : (c) Andrea Rossato
-- License : BSD-style (see LICENSE)
--
-- Maintainer : Andrea Rossato <andrea.rossato@unitn.it>
-- 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
|
