Merge pull request #18 from pseudonom/master

Added filterM

README.md

@@ -297,6 +297,19 @@ unless :: forall m. (Monad m) => Boolean -> m Unit -> m Unit
 ```
 
 
+#### `filterM`
+
+``` purescript
+filterM :: forall a m. (Monad m) => (a -> m Boolean) -> [a] -> m [a]
+```
+
+Filter where the predicate returns a monadic Boolean. For example: 
+
+```purescript
+powerSet :: forall a. [a] -> [[a]]
+powerSet = filterM (const [true, false])
+```
+
 
 ## Module Control.MonadPlus
 

src/Control/Monad.purs

@@ -18,3 +18,18 @@ when false _ = return unit
 unless :: forall m. (Monad m) => Boolean -> m Unit -> m Unit
 unless false m = m
 unless true _ = return unit
+
+-- | Filter where the predicate returns a monadic Boolean. For example: 
+-- |
+-- | ```purescript
+-- | powerSet :: forall a. [a] -> [[a]]
+-- | powerSet = filterM (const [true, false])
+-- | ```
+filterM :: forall a m. (Monad m) => (a -> m Boolean) -> [a] -> m [a]
+filterM _ [] = return []
+filterM p (x:xs) = do
+  b <- p x
+  xs' <- filterM p xs
+  return $ if b
+           then x : xs'
+           else xs'