iterateN (#20)

* iterateN

Create an Unfoldable by repeated application of a function to a seed value.

* Port iterateN to Unfoldable1 and re-export

* Update changelog

Co-authored-by: JordanMartinez <[email protected]>
Co-authored-by: Jordan Martinez <[email protected]>

CHANGELOG.md

@@ -8,6 +8,7 @@ Breaking changes:
 - Migrate FFI to ES modules (#37 by @kl0tl and @JordanMartinez)
 
 New features:
+- Add `iterateN` function (#20 by @matthewleon and @JordanMartinez)
 
 Bugfixes:
 

src/Data/Unfoldable.purs

@@ -18,7 +18,7 @@ import Prelude
 import Data.Maybe (Maybe(..), isNothing, fromJust)
 import Data.Traversable (class Traversable, sequence)
 import Data.Tuple (Tuple(..), fst, snd)
-import Data.Unfoldable1 (class Unfoldable1, unfoldr1, singleton, range, replicate1, replicate1A)
+import Data.Unfoldable1 (class Unfoldable1, unfoldr1, singleton, range, iterateN, replicate1, replicate1A)
 import Partial.Unsafe (unsafePartial)
 
 -- | This class identifies (possibly empty) data structures which can be

src/Data/Unfoldable1.purs

@@ -4,6 +4,7 @@ module Data.Unfoldable1
   , replicate1A
   , singleton
   , range
+  , iterateN
   ) where
 
 import Prelude
@@ -111,3 +112,20 @@ range start end =
     go delta i =
       let i' = i + delta
       in Tuple i (if i == end then Nothing else Just i')
+
+-- | Create an `Unfoldable1` by repeated application of a function to a seed value.
+-- | For example:
+-- |
+-- | ``` purescript
+-- | (iterateN 5 (_ + 1) 0 :: Array Int) == [0, 1, 2, 3, 4]
+-- | (iterateN 5 (_ + 1) 0 :: NonEmptyArray Int) == NonEmptyArray [0, 1, 2, 3, 4]
+-- |
+-- | (iterateN 0 (_ + 1) 0 :: Array Int) == [0]
+-- | (iterateN 0 (_ + 1) 0 :: NonEmptyArray Int) == NonEmptyArray [0]
+-- | ```
+iterateN :: forall f a. Unfoldable1 f => Int -> (a -> a) -> a -> f a
+iterateN n f s = unfoldr1 go $ Tuple s (n - 1)
+  where
+  go (Tuple x n') = Tuple x
+    if n' > 0 then Just $ Tuple (f x) $ n' - 1
+    else Nothing

test/Main.purs

@@ -61,6 +61,9 @@ main = do
   assert $ U1.range 0 0 == NonEmpty 0 []
   assert $ U1.range 0 2 == NonEmpty 0 [1, 2]
 
+  log "Test iterateN"
+  assert $ U.iterateN 5 (_ + 1) 0 == [0, 1, 2, 3, 4]
+
   log "Test Maybe.toUnfoldable"
   assert $ U.fromMaybe (Just "a") == ["a"]
   assert $ U.fromMaybe (Nothing :: Maybe String) == []