Simplifying recursive updating of records - elm

Question

Can this code be simplified?
update : Action -> Model -> Model
update action model =
let
formValue = model.formValue
in
case action of
UpdateWhat what ->
let
newValue = { formValue | what <- what }
in
{ model | formValue <- newValue }
UpdateTrigger trigger ->
let
newValue = { formValue | trigger <- trigger }
in
{ model | formValue <- newValue }
As I plan to add a couple more of Update... clauses, it is helpful to abstract this out a bit.
The code is written the way it is because Elm does not accept inner record updates.

Answer 1

I think what you're looking for is the focus library:
Focus
A Focus is a way to work with particular parts of a large chunk of data. On the most basic level, it lets you get and set fields of a record in a simple and composable way. This means you could avoid writing special record update syntax and use something that composes much more elegantly.
It gives you the ability to write stuff like freeze in the following snippet:
mario =
{ super = False
, fire = False
, physics = { position = { x=3, y=4 }
, velocity = { x=1, y=1 }
}
}
freeze object =
set (physics => velocity) { x=0, y=0 } object
In the code example physics and velocity are Foci. You can create a focus with code like the following, to use your example:
formValue = Focus.create .formValue (\f r -> { r | formValue <- f r.formValue })
what = Focus.create .what (\f r -> { r | what <- f r.what })
trigger = Focus.create .trigger (\f r -> { r | trigger <- f r.trigger })
update : Action -> Model -> Model
update action model =
case action of
UpdateWhat w -> Focus.set (formValue => what) w model
UpdateTrigger t -> Focus.set (formValue => trigger) t model