Question

我有一个描述OrderItem的记录类型：


type alias Product {- = some record type -}
type alias Quantity = Int

type alias OrderItem = {
      product: Product
    , quantity: Quantity
    , fillable: Maybe Bool
}

执行产品查找的功能：

productByCode : String -> Maybe Product

和构建orderItem的函数：

orderItem : String -> Quantity -> Result OrderItem
orderItem productCode quantity =
  let
    product = productByCode productCode
  in
    case product of
      Just p -> Ok { product = p, quantity = quantity, fillable = Nothing }
      Nothing -> Err ("Unknown product code: " ++ productCode)

但是这个函数会导致编译错误：

-- TYPE MISMATCH ---------------------------------------------- src/elm/Main.elm

The type annotation for `orderItem` does not match its definition.

65│ orderItem : String -> Quantity -> Result OrderItem
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The type annotation is saying:

    String -> Quantity -> Result OrderItem

But I am inferring that the definition has this type:

    String
    -> Quantity
    -> Result
           String
           { fillable : Maybe a, product : Product, quantity : Quantity }

错误告诉我编译器没有匹配OrderItem类型，因为Nothing不匹配Maybe Bool - 但肯定Nothing在这里是有效值吗？文档似乎允许以这种方式使用它。

我哪里出错？

Answer 1

类型注释应为：

orderItem : String -> Quantity -> Result String OrderItem

这告诉编译器该值的类型为Ok OrderItem或Err String

`Nothing`与`Maybe a`类型的记录字段不匹配

1 个答案: