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Diffstat (limited to 'src/Runnable.hs')
-rw-r--r-- | src/Runnable.hs | 59 |
1 files changed, 59 insertions, 0 deletions
diff --git a/src/Runnable.hs b/src/Runnable.hs new file mode 100644 index 0000000..56fedb3 --- /dev/null +++ b/src/Runnable.hs @@ -0,0 +1,59 @@ +{-# 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 |