`cata` and `transCata`

Main.purs

module Main where

import Prelude
import Matryoshka

import Effect (Effect)
import Effect.Console (logShow)
import Data.Functor.Mu (Mu(..))

data ListF a n = NilF | ConsF a n

derive instance Functor (ListF a)

instance (Show a, Show n) => Show (ListF a n) where
  show NilF = "NilF"
  show (ConsF h t) = "(ConsF " <> show h <> " " <> show t <> ")"

type List a = Mu (ListF a)

nil :: forall a. List a
nil = In NilF

cons :: forall a. a -> List a -> List a
cons a n = In (ConsF a n)

mapShow :: List Int -> List String
mapShow = cata go
  where
  go NilF = nil
  go (ConsF h t) = cons (show h) t

mapShow' :: List Int -> List String
mapShow' = transCata go
  where
  go NilF = NilF
  go (ConsF h t) = ConsF (show h) t

xs :: List Int
xs = cons 1 (cons 2 (cons 3 (nil)))

main :: Effect Unit
main = do
  logShow (mapShow xs)
  logShow (mapShow' xs)