From 57edbbf2f407433ffb55996a6c2cafbd65bca4fc Mon Sep 17 00:00:00 2001
From: Gary Burgess <gary.burgess@gmail.com>
Date: Fri, 30 Sep 2016 12:26:45 +0100
Subject: [PATCH] Add whenM and unlessM

---
 src/Control/Monad.purs | 17 +++++++++++++++++
 src/Prelude.purs       |  2 +-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/Control/Monad.purs b/src/Control/Monad.purs
index b548721d..497c7c71 100644
--- a/src/Control/Monad.purs
+++ b/src/Control/Monad.purs
@@ -2,6 +2,8 @@ module Control.Monad
   ( class Monad
   , liftM1
   , ap
+  , whenM
+  , unlessM
   , module Data.Functor
   , module Control.Apply
   , module Control.Applicative
@@ -13,6 +15,7 @@ import Control.Apply (class Apply, apply, (*>), (<*), (<*>))
 import Control.Bind (class Bind, bind, ifM, join, (<=<), (=<<), (>=>), (>>=))
 
 import Data.Functor (class Functor, map, void, ($>), (<#>), (<$), (<$>))
+import Data.Unit (Unit)
 
 -- | The `Monad` type class combines the operations of the `Bind` and
 -- | `Applicative` type classes. Therefore, `Monad` instances represent type
@@ -61,3 +64,17 @@ ap f a = do
   f' <- f
   a' <- a
   pure (f' a')
+
+-- | Perform a monadic action when a condition is true, where the conditional
+-- | value is also in a monadic context.
+whenM :: forall m. Monad m => m Boolean -> m Unit -> m Unit
+whenM mb m = do
+  b <- mb
+  when b m
+
+-- | Perform a monadic action unless a condition is true, where the conditional
+-- | value is also in a monadic context.
+unlessM :: forall m. Monad m => m Boolean -> m Unit -> m Unit
+unlessM mb m =  do
+  b <- mb
+  unless b m
diff --git a/src/Prelude.purs b/src/Prelude.purs
index d6813c98..0484a9c6 100644
--- a/src/Prelude.purs
+++ b/src/Prelude.purs
@@ -30,7 +30,7 @@ import Control.Applicative (class Applicative, pure, liftA1, unless, when)
 import Control.Apply (class Apply, apply, (*>), (<*), (<*>))
 import Control.Bind (class Bind, bind, ifM, join, (<=<), (=<<), (>=>), (>>=))
 import Control.Category (class Category, id)
-import Control.Monad (class Monad, ap, liftM1)
+import Control.Monad (class Monad, ap, liftM1, unlessM, whenM)
 import Control.Semigroupoid (class Semigroupoid, compose, (<<<), (>>>))
 
 import Data.Boolean (otherwise)