X.Prelude: Add findM

Several definitions of this were scattered over a few modules, so just
re-export it from our prelude.

XMonad/Actions/TagWindows.hs

@@ -134,11 +134,6 @@ focusTagged' :: (WindowSet -> [Window]) -> String -> X ()
 focusTagged' wl t = gets windowset >>= findM (hasTag t) . wl >>=
     maybe (return ()) (windows . focusWindow)
 
-findM :: (Monad m) => (a -> m Bool) -> [a] -> m (Maybe a)
-findM _ []      = return Nothing
-findM p (x:xs)  = do b <- p x
-                     if b then return (Just x) else findM p xs
-
 -- | apply a pure function to windows with a tag
 withTaggedP, withTaggedGlobalP :: String -> (Window -> WindowSet -> WindowSet) -> X ()
 withTaggedP       t f = withTagged'       t (winMap f)

XMonad/Layout/IM.hs

@@ -30,10 +30,11 @@ module XMonad.Layout.IM (
 ) where
 
 import XMonad
-import qualified XMonad.StackSet as S
 import XMonad.Layout.Grid
 import XMonad.Layout.LayoutModifier
+import XMonad.Prelude
 import XMonad.Util.WindowProperties
+import qualified XMonad.StackSet as S
 
 import Control.Arrow (first)
 
@@ -110,11 +111,6 @@ applyIM ratio prop wksp rect = do
             return (first ((w, masterRect) :) wrs)
         Nothing -> runLayout wksp rect
 
--- | Like find, but works with monadic computation instead of pure function.
-findM :: Monad m => (a -> m Bool) -> [a] -> m (Maybe a)
-findM _ [] = return Nothing
-findM f (x:xs) = do { b <- f x; if b then return (Just x) else findM f xs }
-
 -- | This is for compatibility with old configs only and will be removed in future versions!
 data IM a = IM Rational Property deriving (Read, Show)
 instance LayoutClass IM Window where

XMonad/Prelude.hs

@@ -24,6 +24,7 @@ module XMonad.Prelude (
     notEmpty,
     safeGetWindowAttributes,
     mkAbsolutePath,
+    findM,
 
     -- * Keys
     keyToString,
@@ -89,6 +90,21 @@ chunksOf i xs = chunk : chunksOf i rest
 (.:) :: (a -> b) -> (c -> d -> a) -> c -> d -> b
 (.:) = (.) . (.)
 
+-- | Like 'find', but takes a monadic function instead; retains the
+-- short-circuiting behaviour of the non-monadic version.
+--
+-- For example,
+--
+-- > findM (\a -> putStr (show a <> " ") >> pure False) [1..10]
+--
+-- would print "1 2 3 4 5 6 7 8 9 10" and return @Nothing@, while
+--
+-- > findM (\a -> putStr (show a <> " ") >> pure True) [1..10]
+--
+-- would print @"1"@ and return @Just 1@.
+findM :: Monad m => (a -> m Bool) -> [a] -> m (Maybe a)
+findM p = foldr (\x -> ifM (p x) (pure $ Just x)) (pure Nothing)
+
 -- | 'Data.List.NonEmpty.fromList' with a better error message. Useful to
 -- silence GHC's Pattern match(es) are non-exhaustive warning in places where
 -- the programmer knows it's always non-empty, but it's infeasible to express