How to represent many subsets of an enum?

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Ah that is essentially what we see in elm-css, right?

In that,

-- Css.Structure
type Compatible =
    Compatible

is used as a static tag, and functions return records with .value and many “compatibility” fields. For example px returns records compatible against .length, .calc, .fontSize and so on. While functions such as fontSize takes

type alias FontSize compatible =
    { compatible | value : String, fontSize : Compatible }

so that they only take values with .fontSize : Compatible in them.

I was really amazed by how strong it is to express such kind of compatibility in type-safe manner, with Elm’s type system.