ELM QueryString parser dont compile

问题

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

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

page comes from here

routeParser comes 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")

The error i got

-- 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!

回答1:

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.

来源：https://stackoverflow.com/questions/62254561/elm-querystring-parser-dont-compile

标签

functional-programming

elm