Unfoldable.iterate

Author: matthewleon

bower.json

@@ -18,7 +18,8 @@
   ],
   "dependencies": {
     "purescript-partial": "^1.2.0",
-    "purescript-tuples": "^4.0.0"
+    "purescript-tuples": "^4.0.0",
+    "purescript-control": "^3.3.1"
   },
   "devDependencies": {
     "purescript-assert": "^3.0.0",

src/Data/Unfoldable.purs

@@ -10,12 +10,14 @@ module Data.Unfoldable
   , replicateA
   , none
   , singleton
+  , iterate
   , range
   , fromMaybe
   ) where
 
 import Prelude
 
+import Control.Lazy (class Lazy)
 import Data.Maybe (Maybe(..), isNothing, fromJust)
 import Data.Traversable (class Traversable, sequence)
 import Data.Tuple (Tuple(..), fst, snd)
@@ -89,6 +91,15 @@ none = unfoldr (const Nothing) unit
 singleton :: forall f a. Unfoldable f => a -> f a
 singleton = replicate 1
 
+-- | Lazily produce an Unfoldable structure by iterating a function.
+-- | For example, to lazily produce integers starting with zero:
+-- |
+-- | ~~~ purescript
+-- | nats = iterate add zero
+-- | ~~~
+iterate :: forall f a. Unfoldable f => Lazy (f a) => (a -> a) -> a -> f a
+iterate f = unfoldr (\x -> Just (Tuple x (f x)))
+
 -- | Create an Unfoldable containing a range of values, with both endpoints.
 range :: forall f. Unfoldable f => Int -> Int -> f Int
 range start end =