Question

我很难尝试解析像

这样的结构

{"tree": [5, [[1, 4], [2, 3]]]}

进入二叉树

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

但我似乎无法让这些类型正确。这个问题有一个简单的解决方案吗？

我假设每个JSON数组恰好包含两个元素，因此结果应为

Node (Leaf 5) (Node (Node (Leaf 1) (Leaf 4)) (Node (Leaf 2) (Leaf 3))) :: Tree Int

修改：我尝试过：

我尝试添加新数据类型以便导入

data IntTree = IntTree { jsonTree :: Tree Int }

instance FromJSON IntTree
  where
    parseJSON (Object v) = do
        inttree <- (v .: "tree")
        -- now I am stuck

Answer 1

当我开始使用Aeson时，我发现FromJSON和ToJSON类型很有用，但并不真正了解它们是如何工作的。我发现弄乱underlying types that Aeson uses to represent JSON values可能很有教育意义。例如，它对对象使用HashMap，对数组使用Vector：

{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson
import qualified Data.HashMap.Strict as H
import qualified Data.Vector as V

j = "{\"tree\": [5, [[1, 4], [2, 3]]]}"

data Tree a = Leaf a | Node (Tree a) (Tree a) deriving (Show)

Just (Object decoded) = decode j :: Maybe Value

Just tree = H.lookup "tree" decoded

parse :: FromJSON a => Value -> Tree a
parse t = case t of
    Array v -> if V.length v /= 2
                   then error "Not a binary tree"
                   else Node (parse (V.head v)) (parse (V.last v))
    x       -> Leaf $ case fromJSON x of
                   Success a   -> a
                   Error   msg -> error msg


main = print $ show $ (parse tree :: Tree Int)

Aeson匹配二进制树

1 个答案: