254 lines
8.6 KiB
Scheme
254 lines
8.6 KiB
Scheme
(module control mzscheme
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(provide abort
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fcontrol %
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control prompt control-at prompt-at
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;; `-at' variations expect a prompt tag
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shift reset shift-at reset-at
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control0 prompt0 control0-at prompt0-at
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shift0 reset0 shift0-at reset0-at
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spawn
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splitter
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new-prompt set cupto)
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;; ----------------------------------------
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(define (abort . vals)
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(abort-current-continuation
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(default-continuation-prompt-tag)
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(lambda () (apply values vals))))
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;; ----------------------------------------
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;; Sitaram, PLDI'93
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;; The `%' here is compable with Sitaram & Felleisen, LSC'90,
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;; since we make the handler optional.
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(define (fcontrol f)
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(call-with-composable-continuation
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(lambda (k)
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(abort-current-continuation
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(default-continuation-prompt-tag)
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f
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k))))
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(define-syntax %
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(syntax-rules ()
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[(_ expr handler)
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(call-with-continuation-prompt
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(lambda () expr)
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(default-continuation-prompt-tag)
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handler)]
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[(_ expr)
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(call-with-continuation-prompt
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(lambda () expr))]))
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;; ----------------------------------------
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;; Predecessors of Sitaram, PLDI'93
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;; Felleisen, Wand, Friedman, & Duba, LFP'88
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;; Instead of `#', we use `prompt' as in Felleisen, POPL'88
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;; (where `control' is called `F')
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;; See also Sitaram and Felleisen, LSC'90
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;; Helpder function: abort-current-continuation/keep-prompt is
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;; like abort-current-continuation, but it always leaves the
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;; prompt in place, independent of the prompt's handler.
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;; This is possible via call/cc (i.e., it must be possible
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;; to abort and keep a prompt, because call/cc needs it).
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(define (abort-current-continuation/keep-prompt tag thunk)
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((call-with-continuation-prompt
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(lambda ()
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((call-with-current-continuation
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(lambda (k) (lambda () k))
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tag)))
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tag)
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thunk))
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;; call-with-control, parameterized over whether to keep the
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;; prompt (if the prompt's handler gives us the option of
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;; removing it). The generated function is the same
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;; as fcontrol when `abort-cc' is `abort-current-continuation'.
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(define (make-call-with-control abort-cc)
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;; Uses call/cc to always keep the enclosing prompt.
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(letrec ([call-with-control
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(case-lambda
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[(f) (call-with-control f (default-continuation-prompt-tag))]
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[(f tag) (call-with-composable-continuation
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(lambda (k)
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(abort-cc
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tag
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(lambda ()
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(f k))))
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tag)])])
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call-with-control))
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(define call-with-control
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(make-call-with-control abort-current-continuation/keep-prompt))
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(define-syntax define-control-macros
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(syntax-rules ()
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[(_ control control-at call-with-control)
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(begin
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(define-syntax (control stx)
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(syntax-case stx ()
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[(control id expr0 expr (... ...))
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(identifier? #'id)
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#'(call-with-control (lambda (id) expr0 expr (... ...)))]))
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(define-syntax (control-at stx)
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(syntax-case stx ()
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[(control-at tag id expr0 expr (... ...))
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(identifier? #'id)
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#'(call-with-control (lambda (id) expr0 expr (... ...)) tag)])))]))
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(define-control-macros control control-at call-with-control)
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(define-syntax define-prompt-macros
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(syntax-rules ()
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[(_ prompt prompt-at call-with-prompt)
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(begin
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(define-syntax prompt
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(syntax-rules ()
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[(prompt expr0 expr (... ...))
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(call-with-prompt (lambda () expr0 expr (... ...)))]))
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(define-syntax prompt-at
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(syntax-rules ()
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[(prompt-at tag expr0 expr (... ...))
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(call-with-prompt (lambda () expr0 expr (... ...)) tag)])))]))
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(define-prompt-macros prompt prompt-at call-with-continuation-prompt)
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;; ----------------------------------------
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;; Danvy & Filinski, LFP'90
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;; call-with-shift, parameterized over whether to keep the prompt
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;; (if the prompt's handler gives us the option of removing it),
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;; and whether the new one is removable:
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(define (make-call-with-shift abort-cc inserted-handler)
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(letrec ([call-with-shift
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(case-lambda
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[(f) (call-with-shift f (default-continuation-prompt-tag))]
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[(f tag)
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(call-with-composable-continuation
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(lambda (k)
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(abort-cc
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tag
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(lambda ()
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(f (lambda vals
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(call-with-continuation-prompt
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(lambda ()
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(apply k vals))
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tag
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inserted-handler))))))
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tag)])])
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call-with-shift))
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(define call-with-shift
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(make-call-with-shift abort-current-continuation/keep-prompt #f))
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(define-control-macros shift shift-at call-with-shift)
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(define-prompt-macros reset reset-at call-with-continuation-prompt)
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;; ----------------------------------------
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;; Shan, SCHEME'04
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;; Kiselyov, Indiana CS TR-611, 2005
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;;
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;; The `control0' and `shift0' here are closer to Kiselyov, in that
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;; `control0' and `shift0' only behave as in Shan when paired with
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;; `prompt0' or `reset0' (which are two names for the same thing).
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;; When paired with `prompt' or `reset' (again, the same thing),
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;; they act like `control' and `shift'.
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;;
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;; This difference is intentional. The programmer that inserts a
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;; prompt should choose whether the current continuation is visible
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;; or not. Note, also, that `control' and `shift' work whether
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;; they're paired with `prompt'/`reset' or `prompt0'/`reset0'.
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(define call-with-control0
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;; Uses abort-current-continuation, so that the prompt
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;; is removed --- if the prompt is willing to be removed.
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(make-call-with-control abort-current-continuation))
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(define call-with-shift0
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;; Uses abort-current-continuation, so that the prompt
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;; is removed --- if the prompt is willing to be removed.
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;; The prompt installed with the captured continuation is
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;; itself willing to be removed.
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(make-call-with-shift abort-current-continuation (lambda (thunk) (thunk))))
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(define-control-macros control0 control0-at call-with-control0)
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(define-control-macros shift0 shift0-at call-with-shift0)
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(define call-with-prompt0
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(case-lambda
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[(thunk) (call-with-prompt0 thunk (default-continuation-prompt-tag))]
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[(thunk tag)
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(call-with-continuation-prompt thunk tag (lambda (thunk) (thunk)))]))
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(define-prompt-macros prompt0 prompt0-at call-with-prompt0)
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(define-prompt-macros reset0 reset0-at call-with-prompt0)
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;; ----------------------------------------
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;; Hieb & Dybvig, PPOPP'90
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(define (spawn f)
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(let ([p (make-continuation-prompt-tag)])
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(call-with-continuation-prompt
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(lambda ()
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(f (lambda (f)
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(call-with-composable-continuation
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(lambda (k)
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(abort-current-continuation
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p
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(lambda ()
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(f (lambda vals
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(call-with-continuation-prompt
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(lambda ()
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(apply k vals))
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p
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(lambda (thunk) (thunk))))))))
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p))))
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p
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(lambda (thunk) (thunk)))))
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;; ----------------------------------------
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;; Queinnec & Serpette, POPL'91
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(define (splitter receiver)
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(let ([p (make-continuation-prompt-tag)])
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(call-with-continuation-prompt
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(lambda ()
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(receiver (lambda (thunk)
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(abort-current-continuation
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p
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thunk))
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(lambda (proc)
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(call-with-composable-continuation
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proc
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p))))
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p
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(lambda (thunk) (thunk)))))
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;; ----------------------------------------
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;; Gunter, Remy, & Rieke, FPLCA'95
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;; Unfortunately, the "prompt"s in Gunter et al. are what
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;; we call "prompt tags". In our terminology, a "prompt"
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;; is a tagged instance in a continuation.
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(define (new-prompt) (make-continuation-prompt-tag))
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(define-syntax set (make-rename-transformer #'prompt0-at))
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(define-syntax cupto (make-rename-transformer #'control0-at))
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;; ----------------------------------------
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)
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