Msg's with extra variables in elm inputs - elm

Question

I am trying to partition my Msg values in Elm (0.18) into different types. I'd like it to conform to this kind of typing:
type MsgSession
= LogIn
| LogOut
type MsgSettings
= SetUsername String
= SetPassword String
type Msg
= SessionMsg MsgSession
| SettingsMsg MsgSettings
...
My update function looks like this:
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
case msg of
SessionMsg sessionMsg ->
sessionUpdate sessionMsg model
SettingsMsg settingsMsg ->
settingsUpdate settingsMsg model
...
...which means that I have to define the child updates as well...
sessionUpdate : MsgSession -> Model -> ( Model, Cmd Msg )
sessionUpdate msg model =
case msg of
LogIn ->
-- do stuff
LogOut ->
-- do stuff
My LogOut event looks like this, and works fine:
button [onClick (SessionMsg LogOut)] [text "Log Out"]
But once there is a variable involved with the event, it doesn't work. I have a similar setup for settingsUpdate:
settingsUpdate : MsgSettings -> Model -> ( Model, Cmd Msg )
settingsUpdate msg model =
case msg of
SetUsername string ->
...
SetPassword string ->
...
But I can't get onInput to send as a variable. For example, this code
input [onInput SettingsMsg SetUsername] []
Yields this error:
Function `onInput` is expecting 1 argument, but was given 2.
483| onInput SettingsMsg SetUsername
Parentheses also don't work.
input [onInput (SettingsMsg SetUsername)] []
yields
The argument to function `SettingsMsg` is causing a mismatch.
483| SettingsMsg SetUsername)
^^^^^^^^^^^
Function `SettingsMsg` is expecting the argument to be:
MsgSettings
But it is:
String -> MsgSettings
What do I have to pass into onInput to make this work?

Answer 1

You should be able to use composition:
input [onInput (SettingsMsg << SetUsername)] []
Which, if you're more comfortable with explicit lambdas, looks like this:
input [onInput (\name -> SettingsMsg (SetUsername name))] []