Unfoldable iterateN vs Unfoldable1 iterateN

Main.purs

module Main where

import Prelude

import Data.Maybe (Maybe(..))
import Data.Tuple (Tuple(..))

import Effect (Effect)
import Effect.Console (log)
import TryPureScript as TryPureScript
import Data.Unfoldable1 (class Unfoldable1, unfoldr1)
import Data.Unfoldable (class Unfoldable, unfoldr)
import Data.Array.NonEmpty (NonEmptyArray)

main :: Effect Unit
main = TryPureScript.render =<< TryPureScript.withConsole do
  log $ show $ ((unfoldableIterateN 4 (_ + 1) 0 ) :: Array Int)
  -- Can't use this because NonEmptyArray can't implement Unfoldable
  -- log $ show $ ((unfoldableIterateN 4 (_ + 1) 0 ) :: NonEmptyArray Int)
  
  log ""
  log $ show $ ((unfoldable1IterateN 0 (_ + 1) 0 ) :: Array Int)
  -- Can use it here because NonEmptyArray does implement Unfoldable1
  log $ show $ ((unfoldable1IterateN 0 (_ + 1) 0 ) :: NonEmptyArray Int)
  
unfoldableIterateN :: forall f a. Unfoldable f => Int -> (a -> a) -> a -> f a
unfoldableIterateN n f s = unfoldr go $ Tuple s n
  where
  go (Tuple x n')
    | n' > 0     = Just $ Tuple x $ Tuple (f x) $ n' - 1
    | otherwise = Nothing

-- This modification (see *) is required since the loop will always succeed
-- at least once when used with `unfoldr1` whereas `unfoldr`
-- can fail in its first loop.
unfoldable1IterateN :: forall f a. Unfoldable1 f => Int -> (a -> a) -> a -> f a
unfoldable1IterateN n f s = unfoldr1 go $ Tuple s (n - 1) -- * n is subtracted by 1 here
  where
  go (Tuple x n')
    | n' > 0     = Tuple x $ Just $ Tuple (f x) $ n' - 1
    | otherwise = Tuple x Nothing