range: unfold a range of values

src/Data/Unfoldable.purs

@@ -10,6 +10,7 @@ module Data.Unfoldable
   , replicateA
   , none
   , singleton
+  , range
   , fromMaybe
   ) where
 
@@ -88,6 +89,11 @@ none = unfoldr (const Nothing) unit
 singleton :: forall f a. Unfoldable f => a -> f a
 singleton = replicate 1
 
+-- | Create an Unfoldable containing a range of values, with both endpoints.
+range :: forall f. Unfoldable f => Int -> Int -> f Int
+range start end =
+  unfoldr (\i -> if i <= end then Just (Tuple i $ i + 1) else Nothing) start
+
 -- | Convert a Maybe to any Unfoldable like lists and arrays.
 fromMaybe :: forall f a. Unfoldable f => Maybe a -> f a
 fromMaybe = unfoldr (\b -> flip Tuple Nothing <$> b)

test/Main.purs

@@ -42,6 +42,11 @@ main = do
     [2,1,1],[2,1,2], [2,2,1],[2,2,2]
   ]
 
+  log "Test range"
+  assert $ U.range 1 0 == []
+  assert $ U.range 0 0 == [0]
+  assert $ U.range 0 2 == [0, 1, 2]
+
   log "Test Maybe.toUnfoldable"
   assert $ U.fromMaybe (Just "a") == ["a"]
   assert $ U.fromMaybe (Nothing :: Maybe String) == []