5

我的目标是在 Haskell 中编写一个程序，该程序采用 json 文件的名称并将其余参数解释为导航该 json 文件并打印导航到的值的路径。问题是因为 JSON 可以包含多种值类型，我不知道如何让 Haskell 的类型系统理解我想要的。这是我无法正确实现的带有“导航”功能的 Haskell 代码：

import qualified Data.Aeson as A
import qualified Data.ByteString.Char8 as BS
import qualified Data.ByteString.Lazy.Char8 as BSL
import Data.List
import Data.Maybe
import System.Environment

parse :: String -> A.Value
parse = fromJust . A.decode . BSL.pack

isInteger xs = case reads xs :: [(Integer, String)] of
    [(_, "")] -> True
    _ -> False

navigate :: A.Value -> String -> String
navigate value [] = value
navigate value [x:xs]
    | isInteger x = ??? -- value is an array, get the xth element of it.
    | otherwise = ??? -- value is an map, x is a key in it.

main :: IO ()
main = do
     [filename:path] <- getArgs
     contents <- readFile filename
     let d = parse contents
     putStrLn (show (navigate d path))

作为参考，下面是用 Python 编写相同程序的方式：

from json import load
from sys import argv    
def navigate(obj, path):
    if not path:
        return obj
    head, tail = path[0], path[1:]
    return navigate(obj[int(head) if head.isdigit() else head], tail)    
if __name__ == '__main__':
    fname, path = argv[1], argv[2:]
    obj = load(open(fname))
    print navigate(obj, path)

该程序将像这样运行：

$ cat data.json
{"foo" : [[1, 2, 3, {"bar" : "barf"}]]} 
$ python showjson.py data.json foo 0 3 bar
barf
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1 回答 1

4

您可以简单地在构造函数上进行模式匹配，A.Value以确定您正在处理的 JSON 对象类型：

import qualified Data.HashMap.Strict as M
import qualified Data.Vector as V
import qualified Data.Text as T

-- ... rest of the code more or less as before ...

navigate :: A.Value -> [String] -> BSL.ByteString
navigate value        []       = A.encode value
navigate (A.Array vs) (x : xs) = navigate (vs V.! read   x) xs
navigate (A.Object o) (x : xs) = navigate (o  M.! T.pack x) xs

注意 的定义A.Value如下：

data Value
  = Object !(HashMap Text Value)
  | Array  !(Vector Value)
  | ...  -- other constructors

因此，代码在向量和哈希映射上navigate使用了查找函数（在这两种情况下都调用）。!该函数read用于在需要时将命令行参数解释为数字（如果不是，则会严重失败），而T.pack将字符串重新解释为 type 的值Text。

于 2012-11-06T12:53:14.047 回答