I haven't read this in detail but I expect it to be the same kind of sealed type that many other languages have. It doesn't cover ad-hoc unions (on the fly from existing types) that are possible in F# (and not many non-FP languages with TypeScript being the most notable that does).
orthoxerox 3 minutes ago [-]
Third paragraph from the top:
> unions enable designs that traditional hierarchies can’t express, composing any combination of existing types into a single, compiler-verified contract.
let_rec 9 minutes ago [-]
> ad-hoc unions (on the fly from existing types) that are possible in F#
Are you sure? This is a feature of OCaml but not F# IIUIR
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> unions enable designs that traditional hierarchies can’t express, composing any combination of existing types into a single, compiler-verified contract.
Are you sure? This is a feature of OCaml but not F# IIUIR