PureScript by Example: Chapter 10 (The Foreign Function Interface) exercise 10.19.5

Tree.purs

module Tree where

import Prelude

import Control.Alt

import Data.Either
import Data.Foreign
import Data.Foreign.Class

data Tree a = Leaf a | Branch (Tree a) (Tree a)

instance treeIsForeign :: (IsForeign a) => IsForeign (Tree a) where
    read value = (Branch <$> readProp "left" value <*> readProp "right" value)
             <|> (Leaf <$> readProp "value" value)

instance showTree :: (Show a) => Show (Tree a) where
    show (Branch l r) = "Tree Left [ " ++ show l ++ " ] Right [ " ++ show r ++ "]"
    show (Leaf a)     = "Leaf " ++ show a