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(define (generator p)
  (set! gc #f) ; Continuation in generator
  (lambda ()
    (call/cc (lambda (return)
               (if gc
                   (gc #f)
                   (p (lambda (value)
                        (call/cc (lambda (cgc)
                                   (set! gc cgc)
                                   (return value))))))))))

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// https://underscore.io/blog/posts/2015/04/23/deriving-the-free-monad.html

// Example of implementing a specific instance of a free monad (for getting and
// setting an integer value) in TypeScript. The type system is not expressive
// enough to express free monads in general, but we can express a specific
// instance.

type AtomicOperation<T> = ["get", (x: number) => T] | ["set", number, T];
type Program<T> = ["return", T] | ["suspend", AtomicOperation<Program<T>>];