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#lang typed/racket/base
(require (for-syntax racket/base))
;; Working around what appears to be a bug in Typed Racket
;; by implementing my own promises.
(provide my-delay my-force MyPromise)
(define-struct: Sentinel ())
(define-struct: (a) MyPromise ([forced? : Boolean]
[thunk : (-> a)]
[val : (U Sentinel a)])
#:mutable)
(define-syntax (my-delay stx)
(syntax-case stx ()
[(_ expr ...)
(syntax/loc stx
(make-MyPromise #f
(lambda () expr ...)
(make-Sentinel)))]))
(: my-force (All (a) (MyPromise a) -> a))
(define (my-force a-promise)
(cond
[(MyPromise-forced? a-promise)
(define val (MyPromise-val a-promise))
(if (Sentinel? val)
(error 'force "Impossible")
val)]
[else
(define val ((MyPromise-thunk a-promise)))
(set-MyPromise-val! a-promise val)
(set-MyPromise-forced?! a-promise #t)
val]))
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