Does elm share objects or duplicate them? - elm

Question

I'm working on a e-commerce website with a front-end in Elm and I am wondering if objects are shared or duplicated. I'm pretty sure of the answer but I just want to make sure.
Basically, I have a ProductVariant containing some Colour. At the moment ProductVariant as a field colour_ids : List ColourId (which is what I got from the json) but I am thinking replacing the id with the colour itself : colours : List Colour . I can do that when I decode the JSON and then it's done, I don't need to lookup coulours in the colours dictionary. Am I correct to assume that each Colour will be shared between different variant or each colour will be duplicated, thus taking more memory.

Answer 1

I made a simple program to see the compiled JS output. With Elm code
type alias Color = { red: Int, green: Int, blue: Int }
black: Color
black = { red = 255, green = 255, blue = 255 }
white: Color
white = { red = 0, green = 0, blue = 0 }
red: Color
red = { red = 255, green = 0, blue = 0 }
colors: List Color
colors =
[
black,
white,
red,
{ red = 123, green = 234, blue = 11 },
black,
{ red = 123, green = 234, blue = 11 },
red,
{ red = 123, green = 234, blue = 11 }
]
The output JS code contains
var author$project$SharedData$colors = _List_fromArray(
[
author$project$SharedData$black,
author$project$SharedData$white,
author$project$SharedData$red,
{blue: 11, green: 234, red: 123},
author$project$SharedData$black,
{blue: 11, green: 234, red: 123},
author$project$SharedData$red,
{blue: 11, green: 234, red: 123}
]);
This shows that the compiler is able to reuse the pre-defined colors black, white and red, but when creating a new record even with the exact same data, there'll always be a duplication.
I don't know how your data is organized, but for the above example case, instead of trying to optimize the data structure, I would simply store the colors as hex code strings.
I'm not 100% sure how JS engines handle strings, but on many other platforms there's only one instance in heap of a single string. For the above toy app this would mean to use "7BEA0B" instead of { red: 123, green: 234, blue: 11 }

Answer 2

following #kaskelotti advices I made small program fetching the same item twice from a dictionary and checked using the JS debugger if data where shared.
import Html exposing (text,div)
import Dict
a = {t= "My name is a"}
b = {t= "My name is B"}
d = Dict.fromList [("a", a), ("b", b)]
mytest = List.filterMap (\key -> Dict.get key d) ["a", "b", "a"]
main =
let x = mytest
(y, z)= case x of
[a1,_,a2] -> (a1,a2)
_ -> (a,b)
in div [] (List.map (\o -> text o.t) mytest)
By setting a breakpoint in main I could check that y andz are actually the same (y === z is true). Also modify y.t modifies z.t.