From 23a3b81bfd480e5a1d63bbad91e2b611b99758e1 Mon Sep 17 00:00:00 2001
From: Martin Perner <martin@perner.cc>
Date: Tue, 30 Aug 2011 16:38:04 +0200
Subject: catch alsa exception on decibel acquire

This closes #17.

For some reasons the alsa interface does not provide a dB value on all
systems. Instead an error is returned.
The alsa-mixer library checks for such return values and throws an
exception if one is detected.

This patch catches the exception and returns 0 as dB value.
---
 README                         | 8 +++++---
 src/Plugins/Monitors/Volume.hs | 5 ++++-
 xmobar.cabal                   | 1 +
 3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/README b/README
index caaa882..7acb0c8 100644
--- a/README
+++ b/README
@@ -603,10 +603,12 @@ Monitors have default aliases.
     - `--lowd` _number_ Low threshold for dB. Defaults to -30.0.
 - Variables that can be used with the `-t`/`--template` argument:
             `volume`, `volumebar`, `dB`, `status`
+- Note that `dB` might only return 0 on your system. This is known
+  to happen on systems with a pulseaudio backend.
 - Default template: `Vol: <volume>% <status>`
-- Requires the package [alsa-mixer] installed in your system. In addition,
-  to activate this plugin you must pass `--flags="with_alsa"` during
-  compilation.
+- Requires the package [alsa-core] and [alsa-mixer] installed in your
+  system. In addition, to activate this plugin you must pass
+  `--flags="with_alsa"` during compilation.
 
 `MPD Args RefreshRate`
 
diff --git a/src/Plugins/Monitors/Volume.hs b/src/Plugins/Monitors/Volume.hs
index 50a6ed4..3e3a8b9 100644
--- a/src/Plugins/Monitors/Volume.hs
+++ b/src/Plugins/Monitors/Volume.hs
@@ -14,10 +14,12 @@
 
 module Plugins.Monitors.Volume (runVolume, volumeConfig) where
 
+import Prelude hiding ( catch )
 import Control.Monad ( liftM, mplus )
 import Data.Maybe
 import Plugins.Monitors.Common
 import Sound.ALSA.Mixer
+import Sound.ALSA.Exception ( catch )
 import System.Console.GetOpt
 
 volumeConfig :: IO MConfig
@@ -115,7 +117,8 @@ runVolume mixerName controlName argv = do
         maybeNA = maybe (return "N/A")
     (lo, hi) <- io $ getRange volumeControl
     val <- io $ getChannel FrontLeft $ value volumeControl
-    db <- io $ getChannel FrontLeft $ dB volumeControl
+    db <- io $ catch (getChannel FrontLeft $ dB volumeControl)
+                     (\_ -> return $ Just 0)
     sw <- io $ getChannel FrontLeft switchControl
     p <- maybeNA (formatVol lo hi) val
     b <- maybeNA (formatVolBar lo hi) val
diff --git a/xmobar.cabal b/xmobar.cabal
index ecafe1d..5d77997 100644
--- a/xmobar.cabal
+++ b/xmobar.cabal
@@ -122,6 +122,7 @@ executable xmobar
 
     if flag(with_alsa) || flag(all_extensions)
        build-depends: alsa-mixer == 0.1.*
+       build-depends: alsa-core == 0.5.*
        other-modules: Plugins.Monitors.Volume
        cpp-options: -DALSA
 
-- 
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