Elm parser loop example

Main.elm

module Main exposing (main)

import Browser
import Html exposing (Html, button, div, h2, input, text)
import Html.Attributes exposing (class, placeholder, value)
import Html.Events exposing (onClick, onInput)
import Parser exposing (..)
import Parser as P exposing (..)


type Atom
    = Foo 
    | Bar 
    | Eee
    | Nnn
    | Unknown


main =
    Html.text <|
        Debug.toString <|
            run urlParser "/foo/foo/bar/eee/nnn/x:0x9"


urlParser : Parser (List Atom)
urlParser =
    loop [] fragmentsStep
        |> P.map (List.map <| P.run atom)
        |> P.map (List.map Result.toMaybe)
        |> P.map (List.filterMap identity)


atom : Parser Atom
atom =
    P.oneOf
        [ P.succeed Foo |. P.symbol "foo"
        , P.succeed Bar |. P.symbol "bar"
        , P.succeed Eee |. P.symbol "eee"
        , P.succeed Nnn |. P.symbol "nnn"
        , P.succeed Unknown
        ]


fragmentsStep : List String -> Parser (Step (List String) (List String))
fragmentsStep stmt =
    let
        isAtomChar c = Char.isAlphaNum c || c == ':'
        f acc num =
            if String.length num > 0 then
                Loop (num :: acc)

            else
                Done (List.reverse acc)
    in
    oneOf
        [ succeed (f stmt)
            |. symbol "/"
            |= (chompWhile isAtomChar 
                |> getChompedString)
        , succeed (f stmt)
            |= (chompWhile isAtomChar 
                |> getChompedString)
        ]