match.ts

type Protocol = {
    foo: { x: string };
    bar: { y: number };
}
type Fold<T, U> = {
    readonly [P in keyof T]: (value: T[P]) => U
}
type WithTag<T> = {
    [TKey in keyof T]: { type: TKey } & T[TKey]
}
type ToDiscriminatedUnion<P> = WithTag<P>[keyof P]
const match: <P>(value: ToDiscriminatedUnion<P>) => <R>(f: Fold<P, R>) => R =
    value => f => f[value.type](value)
const res = match<Protocol>({ type: 'foo', x: '1' })({
    foo: ({ x }) => x.length,
    bar: ({ y }) => y,
})
alert(res)

Repl: here