use of existential types for plugin support

This patch, which *changes the configuration format*, adds easy plugin support
by using an existential type for storing the list of commands to be
executed. Adding a plugin is just a matter of writing the appropriate
instance of the Exec class, after importing Commands.hs.
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

darcs-hash:20070712175034-d6583-f10174bb3b0a9b4f6e08d05052c18f30e539b319.gz

1 files changed, 11 insertions, 0 deletions

diff --git a/Runnable.hs-boot b/Runnable.hs-boot
new file mode 100644
index 0000000..90dbe81
--- /dev/null
+++ b/Runnable.hs-boot
@@ -0,0 +1,11 @@
+{-# OPTIONS -fglasgow-exts #-}
+module Runnable where
+import Commands
+
+data Runnable = forall r . (Exec r,Show r, Read r) => Run r
+
+instance Read Runnable
+instance Exec Runnable
+instance Show Runnable
+
+