ELM QueryString 解析器不编译

Question

我真的很想学习一些 ELM，但我的思绪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：给定一个查询字符串 ?name=Neuber 像这样的函数 getParam "name" 会 return 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")

我得到的错误

-- 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 return 变成 Maybe Int，但您正试图将它与 Result.withDefault 一起使用，正如错误消息所述，需要 Result。解决方法是改用 Maybe.withDefault。

ELM QueryString 解析器不编译

ELM QueryString parser dont compile

functional-programming

elm