Purescript Simple JSON，结合记录类型

Question

我是Purescript的新手，并尝试使用purescript-simple-json：

1. 拿一些JSON并将其转换为User的记录
2. 拿一些JSON并将其转换为Comment记录数组
3. 取结果2.并将其放入从1生成的用户记录的comments字段中。

以下是我对此的尝试：

module Test where

import Prelude

import Data.Either (Either)
import Data.Foreign (ForeignError)
import Data.Generic.Rep as Rep
import Data.Generic.Rep.Show (genericShow)
import Data.List.NonEmpty (NonEmptyList)
import Simple.JSON (class ReadForeign, readJSON)

newtype Comment = Comment
  { rating :: Int
  , username :: String
  }

newtype User = User
  { username :: String
  , comments :: Array Comment
  }

derive instance repGenericUser :: Rep.Generic User _
instance showUser :: Show User where show = genericShow

derive instance repGenericComment :: Rep.Generic Comment _
instance showComment :: Show Comment where show = genericShow

derive newtype instance rfUser :: ReadForeign User
derive newtype instance rfComment :: ReadForeign Comment

parseU :: String -> Either (NonEmptyList ForeignError) User
parseU xs = readJSON xs

parseC :: String -> Either (NonEmptyList ForeignError) Comment
parseC xs = readJSON xs

addComments :: User -> Array Comment -> User
addComments (User u) cs = u { comments = cs }

但是，当我尝试使用pulp repl在REPL中加载它时，我收到以下错误消息：

Compiling Test
Error found:
in module Test
at src/test.purs line 39, column 1 - line 39, column 45

  Could not match type

    { comments :: Array Comment
    , username :: String       
    }                          

  with type

    User


while checking that type { comments :: Array Comment
                         | t0                       
                         }                          
  is at least as general as type User
while checking that expression $0 { comments = cs
                                  }              
  has type User
in value declaration addComments

where t0 is an unknown type

我不知道如何解释此错误消息。我可以删除addComments的类型签名，但推断的类型然后变得与上面的错误消息中的未命名记录相同，这不是期望的结果。对我做错了什么的想法？非常感谢你看看这个！

Answer 1

我想你差不多了。错误告诉您正在返回“解包”记录而不是User值。尝试在最后一个函数中添加User构造函数：

addComments :: User -> Array Comment -> User
addComments (User u) cs = User $ u { comments = cs }