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unsplash / sum-types

Safe, ergonomic, non-generic sum types in TypeScript.
https://unsplash.github.io/sum-types/
MIT License
42 stars 2 forks source link

Support for generic sum types #28

Open samhh opened 3 years ago

samhh commented 3 years ago

A tracking issue for if this ever becomes viable. In the meantime we should document the challenges faced.

samhh commented 2 years ago

A new issue has arisen with this as of #45. Where the following was (maybe?) working before, the type system is now unable to determine if A constitutes the value of a nullary constructor (comparison against null here):

type X<A> = Sum.Member<'X', A>
const getX = <A>() => Sum.create<X<A>>()

const f = <A>() => {
  const X = getX<A>()
  X.mk.X // Sum.Constructor<X<A>, "X", A>
}

This extends to subtyping constraints. Changing A on f to for example A extends string makes no difference as, as far as the type system is concerned, A could be string & null.

Could this be solved by representing nullary values with a symbol and switching out to null when (de)serialising? Depends how the typechecker treats unique symbols.