elm-pages-v3-beta/examples/hackernews/app/Effect.elm

module Effect exposing (Effect(..), batch, fromCmd, map, none, perform)

import Browser.Navigation
import Http
import Url exposing (Url)


type Effect msg
    = None
    | Cmd (Cmd msg)
    | Batch (List (Effect msg))
    | FetchRouteData
        { body : Maybe { contentType : String, body : String }
        , path : Maybe String
        , toMsg : Result Http.Error Url -> msg
        }


type alias RequestInfo =
    { contentType : String
    , body : String
    }


none : Effect msg
none =
    None


batch : List (Effect msg) -> Effect msg
batch =
    Batch


fromCmd : Cmd msg -> Effect msg
fromCmd =
    Cmd


map : (a -> b) -> Effect a -> Effect b
map fn effect =
    case effect of
        None ->
            None

        Cmd cmd ->
            Cmd (Cmd.map fn cmd)

        Batch list ->
            Batch (List.map (map fn) list)

        FetchRouteData fetchInfo ->
            FetchRouteData
                { body = fetchInfo.body
                , path = fetchInfo.path
                , toMsg = fetchInfo.toMsg >> fn
                }


perform :
    { fetchRouteData :
        { body : Maybe { contentType : String, body : String }
        , path : Maybe String
        , toMsg : Result Http.Error Url -> pageMsg
        }
        -> Cmd msg
    , fromPageMsg : pageMsg -> msg
    , key : Browser.Navigation.Key
    }
    -> Effect pageMsg
    -> Cmd msg
perform ({ fromPageMsg, key } as helpers) effect =
    case effect of
        None ->
            Cmd.none

        Cmd cmd ->
            Cmd.map fromPageMsg cmd

        Batch list ->
            Cmd.batch (List.map (perform helpers) list)

        FetchRouteData fetchInfo ->
            helpers.fetchRouteData
                { body = fetchInfo.body
                , path = fetchInfo.path
                , toMsg = fetchInfo.toMsg
                }