Hm, I sort of ignored these when diving into ES6 at first, but they're perfect for making extensible sum types function match(t, matchers) { const va

Hm, I sort of ignored these when diving into ES6 at first, but they're perfect for making extensible sum types function match(t, matchers) { const variant = t[Symbol.for('variant')]; return matchers[variant](t); } Previously one had to make the sum look something like { _variant: String , _params: Obj } to ensure that you could work generically with them and not have the _params object accidentally clobber your variant. Underscore privates are nice, but... obviously a hack.