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

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

import Browser.Navigation
import Form.FormData exposing (FormData)
import Http
import Json.Decode as Decode
import Pages.Fetcher
import Url exposing (Url)


type Effect msg
    = None
    | Cmd (Cmd msg)
    | Batch (List (Effect msg))
    | GetStargazers (Result Http.Error Int -> msg)
    | SetField { formId : String, name : String, value : String }
    | FetchRouteData
        { data : Maybe FormData
        , toMsg : Result Http.Error Url -> msg
        }
    | Submit
        { values : FormData
        , toMsg : Result Http.Error Url -> msg
        }
    | SubmitFetcher (Pages.Fetcher.Fetcher 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)

        GetStargazers toMsg ->
            GetStargazers (toMsg >> fn)

        FetchRouteData fetchInfo ->
            FetchRouteData
                { data = fetchInfo.data
                , toMsg = fetchInfo.toMsg >> fn
                }

        Submit fetchInfo ->
            Submit
                { values = fetchInfo.values
                , toMsg = fetchInfo.toMsg >> fn
                }

        SetField info ->
            SetField info

        SubmitFetcher fetcher ->
            fetcher
                |> Pages.Fetcher.map fn
                |> SubmitFetcher


perform :
    { fetchRouteData :
        { data : Maybe FormData
        , toMsg : Result Http.Error Url -> pageMsg
        }
        -> Cmd msg
    , submit :
        { values : FormData
        , toMsg : Result Http.Error Url -> pageMsg
        }
        -> Cmd msg
    , runFetcher :
        Pages.Fetcher.Fetcher pageMsg
        -> Cmd msg
    , fromPageMsg : pageMsg -> msg
    , key : Browser.Navigation.Key
    , setField : { formId : String, name : String, value : String } -> Cmd msg
    }
    -> Effect pageMsg
    -> Cmd msg
perform ({ fromPageMsg, key } as helpers) effect =
    case effect of
        None ->
            Cmd.none

        Cmd cmd ->
            Cmd.map fromPageMsg cmd

        SetField info ->
            helpers.setField info

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

        GetStargazers toMsg ->
            Http.get
                { url =
                    "https://api.github.com/repos/dillonkearns/elm-pages"
                , expect = Http.expectJson (toMsg >> fromPageMsg) (Decode.field "stargazers_count" Decode.int)
                }

        FetchRouteData fetchInfo ->
            helpers.fetchRouteData
                fetchInfo

        Submit record ->
            helpers.submit record

        SubmitFetcher record ->
            helpers.runFetcher record