错误无法匹配类型

Question

我有一个类型：

newtype User = User
  { id :: String
  , email :: String
  , last_update :: String
  }

和一个函数：

import Pux.DOM.HTML (HTML)
import Pux.DOM.HTML.Attributes (key)
import Text.Smolder.HTML as H
import Text.Smolder.HTML.Attributes as A

userRow :: User -> HTML Event
  userRow user =
    H.tr ! key user.id ! A.className "user-item" $ do
      H.td ! key ("email") $ text user.email
      H.td ! key ("last_update") $ text user.last_update
      H.td ! key ("actions") ! A.className "actions" $ do
        H.a ! key ("delete")  ! A.className "action-delete" #! onClick (pure $ OpenDeleteModal (show user.id)) $ do
          H.span
          ! A.className "dashicons dashicons-trash"
          ! A.alt "Delete"
          ! A.title "Delete"
          $ text ""

但是类型不会统一:

268    H.tr ! key user.id ! A.className "user-item" $ do
                  ^^^^

Could not match type

  { id :: String
  | t0
  }

with type

  User

while checking that type User
  is at least as general as type { id :: String
                                 | t0
                                 }
while checking that expression user
  has type { id :: String
           | t0
           }
while checking type of property accessor user.id
in value declaration userRow

where t0 is an unknown type

我看不出我做错了什么。在我看来，用户类型有一个 "id" 成员，它是一个字符串，因此至少与

一样通用

type { id :: String
     | t0
     }

Answer 1

User 是围绕一条记录的新类型，因此它将记录包裹起来。为了访问记录字段，您首先需要将其解包。在您的情况下执行此操作的最简单方法是在 user 参数上进行模式匹配：

userRow :: User -> HTML Event
userRow (User user) =
  H.tr ! key user.id ! A.className "user-item" $ do
    ...

错误无法匹配类型

Error could not match type

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