optimize monadic ||

(||) used applicative style runs both conditions rather than short
circuiting. Add an orM that properly short-circuits.

1 files changed, 11 insertions, 1 deletions

diff --git a/Utility/Monad.hs b/Utility/Monad.hs
index 23c0c4c19..5cc243290 100644
--- a/Utility/Monad.hs
+++ b/Utility/Monad.hs
@@ -8,7 +8,7 @@
 module Utility.Monad where
 
 import Data.Maybe
-import Control.Monad (liftM)
+import Control.Monad (liftM, liftM2)
 
 {- Return the first value from a list, if any, satisfying the given
  - predicate -}
@@ -31,6 +31,16 @@ ifM cond (thenclause, elseclause) = do
 	c <- cond
 	if c then thenclause else elseclause
 
+{- monadic ||
+ -
+ - Compare with (||) <$> ma <*> mb, which always runs both ma and mb. -}
+orM :: Monad m => m Bool -> m Bool -> m Bool
+orM ma mb = ifM ma ( return True , mb )
+
+{- monadic &&  (for completeness) -}
+andM :: Monad m => m Bool -> m Bool -> m Bool
+andM  = liftM2 (&&)
+
 {- Runs an action, passing its value to an observer before returning it. -}
 observe :: Monad m => (a -> m b) -> m a -> m a
 observe observer a = do