Add filtered (#9)

* add filtered

Author: safareli

src/Control/Monad/Gen.purs

@@ -103,10 +103,17 @@ unfoldable gen = unfoldr unfold <$> sized (tailRecM loopGen <<< Tuple Nil)
 -- | matches a given predicate. This will never halt if the predicate always
 -- | fails.
 suchThat :: forall m a. MonadRec m => MonadGen m => m a -> (a -> Boolean) -> m a
-suchThat gen pred = tailRecM go unit
+suchThat gen pred = filtered $ gen <#> \a -> if pred a then Just a else Nothing
+
+-- | Creates a generator that repeatedly run another generator until it produces
+-- | `Just` node. This will never halt if the input generatr always produces Nothing.
+filtered :: forall m a b. MonadRec m => MonadGen m => m (Maybe a) -> m a
+filtered gen = tailRecM go unit
   where
   go :: Unit -> m (Step Unit a)
-  go _ = gen <#> \a -> if pred a then Done a else Loop unit
+  go _ = gen <#> \a -> case a of
+    Nothing -> Loop unit
+    Just a -> Done a
 
 fromIndex :: forall f a. Foldable f => Int -> a -> f a -> a
 fromIndex i a = fromMaybe a <<< snd <<< (foldl go (Tuple 0 (Just a)))