ELM QueryString 解析器不编译

Question

我真的很想学习一点 ELM，但我的思想在查询解析时崩溃了，我的想法是创建一个函数来按名称获取查询字符串值，例如：给定查询字符串 ?name=Neuber 一个函数像这样的getParam "name" 会返回Neuber

但它在最基本的例子中失败了，它甚至没有编译

page 来from here

routeParser 来from here

module Main exposing (..)
-- import Url.Parser exposing (Parser, (</>), (<?>), oneOf, s)
import Url.Parser.Query exposing (int, map, map2, string)

type alias QueryParams =
  { search : Maybe String
  , page : Maybe Int
  }


routeParser : Url.Parser.Query.Parser QueryParams
routeParser = map2 QueryParams (string "search") (int "page")

page : Url.Parser.Query.Parser Int
page = map (Result.withDefault 1) (int "page")

我遇到的错误

-- TYPE MISMATCH ---------------- /a/long/way/to/project/src/Main.elm

The 2nd argument to `map` is not what I expect:

15| page = map (Result.withDefault 1) (int "page")
                                       ^^^^^^^^^^
This `int` call produces:

    Url.Parser.Query.Parser (Maybe Int)

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

    Url.Parser.Query.Parser (Result x number)

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!

Answer 1

直接的问题是int "page" 将返回Maybe Int，但您尝试将它与Result.withDefault 一起使用，正如错误消息所述，它需要Result。解决此问题的方法是改用Maybe.withDefault。