类型不匹配（对于自定义类型）

Question

与我用普通英语解释相比，我认为代码更能说明问题。

这是域建模的代码：

module Domain.A exposing (..)

import Domain.B

type Size
  = Pound Float
  | Kilogram Float
  | Gram Float
  | Liter Float
  | Ounce Float
  | Count Int

type StockStatus 
  = InStock
  | OutOfStock

type alias CityWholesalerProduct =
  {
    cityWholesalerId : String,
    brand : String,
    productSku : Maybe String,
    sku : String,
    upc : Maybe String,
    description : String,
    status : StockStatus,
    price : Float,
    unit : Domain.B.Unit,
    pack : Int,
    size : Size
  }

这是Main.elm

中模型的代码

-- MODEL

type alias Model =
  { cityWholesalerProducts : (List Domain.A.CityWholesalerProduct)
  , cityWholesalerProduct : Domain.A.CityWholesalerProduct
  }

init : Model
init = 
  Model 
    [] 
    {
      cityWholesalerId = "",
      brand = "",
      productSku = "",
      sku = "",
      upc = "",
      description = "",
      status = Domain.A.OutOfStock,
      price = 0.00,
      unit = Domain.B.Count,
      pack = 0,
      size = Domain.A.Count 0
    }

问题是我收到一条错误消息：

TYPE MISMATCH - The 2nd argument to `Model` is not what I expect:

34|   Model 
35|     [] 
36|#>#    {
37|#>#      cityWholesalerId = "",
38|#>#      brand = "",
39|#>#      productSku = "",
40|#>#      sku = "",
41|#>#      upc = "",
42|#>#      description = "",
43|#>#      status = Domain.A.OutOfStock,
44|#>#      price = 0.00,
45|#>#      unit = Domain.B.Count,
46|#>#      pack = 0,
47|#>#      size = Domain.A.Count 0
48|#>#    }

This argument is a record of type:

    { brand : String
    , cityWholesalerId : String
    , description : String
    , pack : Int
    , price : Float
    , productSku : #String#
    , size : Domain.A.Size
    , sku : String
    , status : Domain.A.StockStatus
    , unit : Domain.B.Unit
    , upc : #String#
    }

But `Model` needs the 2nd argument to be:

    #Domain.A.CityWholesalerProduct#

#Hint#: I always figure out the argument types from left to right. If an argument
is acceptable, I assume it is “correct” and move on. So the problem may actually
be in one of the previous arguments!

总的来说，我是 Elm 和 FP 的新手，类型不匹配错误是我最大的痛点，我已经在这个问题上停留了一段时间。

Answer 1

令我惊讶的是，Elm 编译器（通常会提供非常有用的错误消息）在这里没有更具体。但我看到的问题只是在那些领域 - productSku 和 upc - 你的类型别名说它们应该是 Maybe String 类型，但你已经用值 "" 初始化了], 这只是一个 String.

不清楚哪一部分是错误的。如果那些总是应该是简单的字符串，那么修复类型声明，在那些地方将 Maybe String 更改为 String。但我假设类型声明可能如您所愿，并且值是错误的。您可以试试这个：

init = 
  Model 
    [] 
    {
      cityWholesalerId = "",
      brand = "",
      productSku = Just "",
      sku = "",
      upc = Just "",
      description = "",
      status = Domain.A.OutOfStock,
      price = 0.00,
      unit = Domain.B.Count,
      pack = 0,
      size = Domain.A.Count 0
    }

这至少应该编译 - 但我无法评论您预期的业务逻辑的正确值是什么。我不清楚 Nothing 对这些 Maybe 值意味着什么，以及这是否真的与 Just "" 有任何不同 - 但很可能它们是。

类型不匹配（对于自定义类型）

Type Mismatch (for a custom type)

functional-programming

elm