Hi, I'm a new comer to elm. I try to build application using elm and have
some problem on component.
Suppose I'm building an BMI calculator, initially I have a model
type alias Model = { weight:Int, height:Int }
The rendering is just an two input[type=number].
The result is rendered by a pure function at the main app:
calclulate model =
let
h = (toFloat model.height) / 100
w = toFloat model.weight
in
toString (round (w / (h*h)))
One day a decision made to change those two inputs into a component of its
own (BmiConfig).
So the models become
--at main app
type alias Model = { bmiConfig: BmiConfig.Model ... }
--at BmiConfig
type alias Model = {weight:Int, height:Int}
We need to to the work of connecting Msg, etc. The calculate function
becomes
calclulate model =
let
h = (toFloat model.config.height) / 100
w = toFloat model.config.weight
in
toString (round (w / (h*h)))
Another a change comes and requesting add +/- button to the number input.
We try to make a small generic component NumberInput. Models become:
--at main app
type alias Model = { bmiConfig: BmiConfig.Model ... }
--at BmiConfig
type alias Model = {weight:NumberInput.Model, height:NumberInput.Model}
--at NumberInput
type alias Model = {value:Int, max:Int, min:Int, .... }
The calculate function needs to be changed again:
calculate model =
let
h = (toFloat model.config.height.value) / 100
w = toFloat model.config.weight.value
in
toString (round (w / (h*h)))
It seems to me that every refactoring to extract component would lead to a
propagation
of change all the ways from child to its parents. Parents need to know the
data
structure of child to do calculation too. Is that the correct way of
applying the elm architecture?
Have I missed something?
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