How to convert Html.Html msg to Html.Html Msg? - elm

Question

I'm trying to write a TabControl in Elm as follows
module TabControl exposing (..)
import Html
import Html.Attributes as Attributes
import Html.Events
import Array
type Msg
= OnSelectedTabChanged
| NoOp
type alias Model msg =
{ tabs : List (Tab msg)
, selectedIndex : Int
}
constructor : Model msg
constructor =
{ tabs = []
, selectedIndex = 0
}
type alias Tab msg =
{ title : String
, content : Html.Html msg
}
withTab : Tab msg -> Model msg -> (Model msg)
withTab tab model =
{ model | tabs = model.tabs ++ [tab] }
render : Model msg -> Html.Html msg
render model =
let
header = renderTabHeaders model
content = renderSelectedTabContent model
in
Html.div [] [ header, content ]
renderTabHeaders : Model msg -> Html.Html msg
renderTabHeaders model =
Html.div []
[
Html.ul []
(
model.tabs
|> List.map (\(tab) -> renderTabHeader tab)
)
]
renderTabHeader : Tab msg -> Html.Html msg
renderTabHeader tab =
Html.li [Html.Events.onClick OnSelectedTabChanged] [Html.text tab.title]
renderSelectedTabContent : Model msg -> Html.Html msg
renderSelectedTabContent model =
let
array =
Array.fromList model.tabs
item =
Array.get model.selectedIndex array
in
case item of
Just value ->
value.content
Nothing ->
Html.text ""
which is rendered by
module Main exposing (..)
import Browser
import Html exposing (Html, button, div, text)
import Html.Events exposing (onClick)
import TabControl
main =
Browser.sandbox { init = 0, update = update, view = view }
view model =
div []
[
TabControl.constructor
|> TabControl.withTab (TabControl.Tab "title 1" (Html.text "html 1"))
|> TabControl.withTab (TabControl.Tab "title 2" (Html.text "html 2"))
|> TabControl.render
]
However, I can't seem to get the return types correct.
If I refactor renderTabHeader : Tab msg -> Html.Html msg to renderTabHeader : Tab msg -> Html.Html Msg then
Something is off with the body of the `renderTabHeaders` definition:
This `div` call produces:
Html.Html Msg
But the type annotation on `renderTabHeaders` says it should be:
Html.Html msg
if I don't then
Something is off with the body of the `renderTabHeader` definition:
This `li` call produces:
Html.Html Msg
But the type annotation on `renderTabHeader` says it should be:
Html.Html msg
If I make every function return Html.Html Msg then
Something is off with the 1st branch of this `case` expression:
67| value.content
^^^^^^^^^^^^^
The value at .content is a:
Html.Html msg
But the type annotation on `renderSelectedTabContent` says it should be:
Html.Html Msg
How can I have elements in a div, some of which return Html.Html msg and some of which return Html.Html Msg? Or alternatively, how can I convert between Html.Html msg and Html.Html Msg

Answer 1

If you're wanting to expose your Msg type, you'll need the user of your module to supply a function which translates your Msg into their msg.
So, if the top-level Msg type looks like this:
type Msg
= NoOp
| TabControlMsg TabControl.Msg
| SomethingElse
Then render can take a function which accepts a TabControl.Msg and returns a Msg, which, in this case, would be the TabControlMsg constructor.
view model =
div []
[
TabControl.constructor
|> TabControl.withTab (TabControl.Tab "title 1" (Html.text "html 1"))
|> TabControl.withTab (TabControl.Tab "title 2" (Html.text "html 2"))
|> TabControl.render TabControlMsg
]
Then you would adjust the code in your module to use that function:
render : (Msg -> msg) -> Model msg -> Html.Html msg
render toMsg model =
let
header = renderTabHeaders model
content = renderSelectedTabContent toMsg model
in
Html.div [] [ header, content ]
renderSelectedTabContent : (Msg -> msg) -> Model msg -> Html.Html msg
renderSelectedTabContent toMsg model =
let
array =
Array.fromList model.tabs
item =
Array.get model.selectedIndex array
in
case item of
Just value ->
Html.map toMsg value.content
Nothing ->
Html.text ""

Answer 2

Ah.. so I can map value.content to return a Msg like so
renderSelectedTabContent : Model msg -> Html.Html Msg
renderSelectedTabContent model =
let
array =
Array.fromList model.tabs
item =
Array.get model.selectedIndex array
in
case item of
Just value ->
Html.map (\_ -> NoOp) value.content
Nothing ->
Html.text ""