From 1c7c4578ec021ec1bbf4ea4efe5be6a7ed5f0cde Mon Sep 17 00:00:00 2001
From: Iustin Pop <iustin@google.com>
Date: Tue, 20 Sep 2011 17:28:49 +0900
Subject: [PATCH] htools: add a MonadPlus instance for Result
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This will be used to implement more easily 'choice' parsing of input
data, without resorting to syntax (case β¦ of Bad _ -> β¦).
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>
---
htools/Ganeti/HTools/Types.hs | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/htools/Ganeti/HTools/Types.hs b/htools/Ganeti/HTools/Types.hs
index 6452f5b6d..ec774b012 100644
--- a/htools/Ganeti/HTools/Types.hs
+++ b/htools/Ganeti/HTools/Types.hs
@@ -67,6 +67,7 @@ module Ganeti.HTools.Types
, EvacMode(..)
) where
+import Control.Monad
import qualified Data.Map as M
import qualified Text.JSON as JSON
@@ -272,6 +273,14 @@ instance Monad Result where
return = Ok
fail = Bad
+instance MonadPlus Result where
+ mzero = Bad "zero Result when used as MonadPlus"
+ -- for mplus, when we 'add' two Bad values, we concatenate their
+ -- error descriptions
+ (Bad x) `mplus` (Bad y) = Bad (x ++ "; " ++ y)
+ (Bad _) `mplus` x = x
+ x@(Ok _) `mplus` _ = x
+
-- | Simple checker for whether a 'Result' is OK.
isOk :: Result a -> Bool
isOk (Ok _) = True
--
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