ELM QueryString解析器不编译 [英] ELM QueryString parser dont compile

问题描述

我实际上是在尝试学习ELM，但是在解析查询时我的头脑崩溃了，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 像这样的函数 getParam name 会返回 Neuber

I am really trying to learn a bit of ELM, but my mind collapse at the query parse, my idea was to create a function to get a query string value by name something like: given an query string ?name=Neuber a function like this getParam "name" that would return Neuber

但是它在最基本的示例中失败了，甚至无法编译

But its failing at most basic example, it doesn't even compile

页面来自来自

routeParser 来自从这里开始

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")

我收到的错误

-- 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!

推荐答案

直接的问题是 int页面 会返回也许是Int ，但是您尝试将其与 Result.withDefault一起使用期望 Result 。解决方法是仅使用 Maybe.withDefault 。

The immediate problem is that int "page" will return a Maybe Int, but you're trying to use it with Result.withDefault, which, as the error message says, expects a Result. The fix for this is just to use Maybe.withDefault instead.

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