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Encoding of existential types in TypeScript
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type StackOps<S, A> = { | |
init(): S | |
push(s: S, x: A): void | |
pop(s: S): A | |
size(s: S): number | |
} | |
type Stack<A> = <R>(go: <S>(ops: StackOps<S, A>) => R) => R | |
const arrayStack = <A>(): Stack<A> => | |
go => go<A[]>({ | |
init: () => [], | |
push: (s, x) => s.push(x), | |
pop: s => { if (s.length) return s.pop()!; else throw Error('empty stack!'); }, | |
size: s => s.length, | |
}) | |
const doStackStuff = (stack: Stack<string>) => | |
stack(({ init, push, pop, size }) => { | |
const s = init() | |
push(s, 'hello') | |
push(s, 'world') | |
push(s, 'omg') | |
pop(s) | |
return size(s) | |
}) | |
expect(doStackStuff(arrayStack())).toBe(2) |
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