haskell - 无法将类型 ‘[Char]’ 与 ‘Data.Text.Internal.Text’ 匹配

Question

我想弄清楚，如何构建 JSON在 Haskell用下面的例子:

module Main where


import GHC.Exts
import Data.Aeson
import qualified Data.Text.Lazy.IO as T
import qualified Data.Text.Lazy.Encoding as T

val :: Value
val = Object $ fromList [
  ("numbers", Array $ fromList [Number 1, Number 2, Number 3]),
  ("boolean", Bool True) ]

main :: IO ()
main = T.putStrLn . T.decodeUtf8 . encode $ val

当我尝试编译时，编译器提示:

    • Couldn't match type ‘[Char]’ with ‘Data.Text.Internal.Text’
      Expected type: Item Object
        Actual type: ([Char], Value)
    • In the expression: ("boolean", Bool True)
      In the first argument of ‘fromList’, namely
        ‘[("numbers", Array $ fromList [Number 1, Number 2, ....]),
          ("boolean", Bool True)]’
      In the second argument of ‘($)’, namely
        ‘fromList
           [("numbers", Array $ fromList [Number 1, Number 2, ....]),
            ("boolean", Bool True)]’
   |
12 |   ("boolean", Bool True) ]
   |   ^^^^^^^^^^^^^^^^^^^^^^

Answer 1

你需要把 OverloadedStrings文件顶部的扩展名:

#!/usr/bin/env stack
-- stack script --resolver lts-12.7
{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson
import qualified Data.Text.Lazy.Encoding as T
import qualified Data.Text.Lazy.IO as T
import GHC.Exts

val :: Value
val =
  Object $
  fromList
    [ ("numbers", Array $ fromList [Number 1, Number 2, Number 3])
    , ("boolean", Bool True)
    ]

main :: IO ()
main = T.putStrLn . T.decodeUtf8 . encode $ val

在执行它们时:

$ stack fuse.hs
{"boolean":true,"numbers":[1,2,3]}

要了解它起作用的原因， refer to this answer .