recursion-schemes example

Mystudies.hs

{-# LANGUAGE DeriveTraversable #-}
{-# LANGUAGE LambdaCase        #-}
{-# LANGUAGE TemplateHaskell   #-}
{-# LANGUAGE TypeFamilies      #-}

module Mystudies where

import           Data.Foldable            (toList)
import           Data.Functor.Foldable    (fold)
import           Data.Functor.Foldable.TH (makeBaseFunctor)
import           Test.QuickCheck          (quickCheckAll, (===))

data Tree a = Leaf a | Node (Tree a) (Tree a)
  deriving (Foldable)
makeBaseFunctor ''Tree

tree :: Tree String
tree =
  Node
    (Node (Leaf "To") (Leaf "iterate"))
    (Node
      (Node (Leaf "is") (Leaf "human,"))
      (Node (Leaf "to") (Node (Leaf "recurse") (Leaf "divine"))))

prop_folding_to_String_1 =
  unwords (toList tree) === "To iterate is human, to recurse divine"

prop_folding_to_String_2 =
  fold (\case LeafF s -> s; NodeF a b -> a ++ " " ++ b) tree
  === "To iterate is human, to recurse divine"

pure []
main = $quickCheckAll

-- $> main