elm - union types and the model

Question

The following code from this guide employs a union type to hold all possible widget types, then render a view:

type Widget
    = ScatterPlot (List (Int, Int))
    | LogData (List String)
    | TimePlot (List (Time, Int))

view : Widget -> Element
view widget =
    case widget of
      ScatterPlot points ->
          viewScatterPlot points

      LogData logs ->
          flow down (map viewLog logs)

      TimePlot occurrences ->
          viewTimePlot occurrences

My rather elementary question is:

With respect to the Widget type, what is the underlying model here?

Given an action (e.g. user wants to see a scatter graph widget), what is the underlying structure that should be updated?

score 2 · Accepted Answer · edited Apr 26 '16 at 20:35

2

Widget is your raw data. You then need to model separately which data to show

type alias Model = Widget

Then suppose that you have some incoming IO data

update action model = 
  case action of 
    ScatterPoints pts ->     -- List (Int, Int)
      ScatterPlot pts
    LogPoints pts ->
      LogData pts
    ...

then having loaded the data in the right type of Widget your view function from the original question will know what to do with the data

edited Apr 26 '16 at 20:35

category

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answered Apr 26 '16 at 19:40

Simon H

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Are permutations of 2 or 3 widget types outside the scope of the example? I was wondering what would need to be updated if the user wanted to switch from one widget type to another. – category Apr 26 '16 at 19:58
You would need a user event to trigger the update cycle, and a function that transforms the data to the new format – Simon H Apr 27 '16 at 05:22

elm - union types and the model

1 Answers1