We present the library as an implementation of an abstract machine derived by elaborating the definitional machine. The abstract view lets us distill a minimalistic API, scAPI, sufficient for implementing multi-prompt delimited control. We argue that a language system that supports exception and stack-overflow handling supports scAPI. With byte- and native-code OCaml systems as two examples, our library illustrates how to use scAPI to implement multi-prompt delimited control in a typed language. The approach is general and has been used to add multi-prompt delimited control to other existing language systems.
Slides of the talk at FLOPS10, April 21, 2010.
shift as a green
The example used in the talk to introduce the delimcc library
The delimcc library distribution
control0delimited continuation operators with multiple, arbitrarily typed prompts. The library has been fruitfully used since 2006, for implementing (delimited) dynamic binding, probabilistic programming, CGI programming with nested transactions, efficient and comprehensible direct-style code generators, normalization of MapReduce-loop bodies by evaluation, and automatic bundling of RPC requests. This web site details these and other examples of using delimcc.
The delimcc library was the first direct implementation of delimited control in a typed, mainstream, mature language -- it captures only the needed prefix of the current continuation, requires no code transformations, and integrates with native-language exceptions. Exception handlers may be captured in delimited continuations (and re-instated when the captured continuation is installed); exceptions remove the prompts along the way. The implementation has no typing problems, no bizarre
'a cont types, and no use for magic.
The delimcc library is a pure library and makes no changes to the OCaml system -- neither to the compiler nor to the run-time system. Therefore the library is perfectly compatible with any OCaml program and any (compiled) OCaml library. The delimcc library has no performance or other effect on the code that does not capture delimited continuations.
The native- and byte-code versions of the library implement the identical interface, described in
delimcc.mli . The two versions share the very same OCaml code. Only the C code files, implementing scAPI, vary between byte- and native-code. Using the native-code delimcc is identical to the byte-code version; the sole change is invoking the ocamlopt compiler to build the project.
The byte-code version of the delimcc library supports serialization of captured continuations. The library defines a convenient debugging primitive
show_val to outline the structure of any OCaml value.
The very operation of capturing and reinstalling a delimited continuation will always be faster in byte-code than in the native code. A captured byte-code continuation is a uniform sequence of values and code pointers. In contrast, the corresponding captured native-code delimited continuation -- a portion of the C stack -- is not only bigger but also contains unboxed values. We have to refer to frame tables to figure out which stack slots contain heap pointers. Manipulating the stack required extra care and effort to please GC. Since the captured native-code continuation contains the mixture of boxed and unboxed values, it is not an ordinary OCaml value, requiring a custom GC scanning function. Custom-scanned data types can be emulated without any changes to OCaml, thanks to GC hooks. Alas, the emulation does not seem to be efficient.
The library has been tested with OCaml 3.08, 3.09, 3.10.2, 3.11, 3.12.0 and 4.00.1 on i386 and amd64 Linux and FreeBSD platforms.
Since OCaml byte-code is portable, the byte-code delimcc should work on any supported architecture. The native-code part can probably be used on other architectures whose stack grows downwards. The library contains no custom assembly code and is written to be a portable client of the OCaml run-time.
The library is distributed under the MIT license.
testd0.mlcontains many simple examples of using delimited control.
R. Kent Dybvig, Simon Peyton Jones, and Amr Sabry: A Monadic Framework for Delimited Continuations.
JFP, v17, N6, pp. 687--730, 2007.
< http://www.cs.indiana.edu/cgi-bin/techreports/TRNNN.cgi?trnum=TR615 >
Delimited Control in OCaml, Abstractly and Concretely
The explanation of the implementation techniques
Persistent twice-delimited continuations
The explanation of the serialization of captured delimited continuations
For historical interest: the original, indirect, monadic-style implementation. Oct 27, 2005.
All three libraries provide monad transformers, with basic operations to capture and reinstall delimited continuations:
takeSubCont/pushSubCont . All three libraries support multiple, typed prompts. All three libraries are quite distinct from the original implementation in Dybvig, Peyton Jones, Sabry's paper. For instance, none of the new libraries use
unsafeCoerce . All three implementations are derived from the specification of delimited control: from the reduction semantics or from the definitional interpreter. The new libraries are faster.
The new libraries differ in: performance; ease of understanding; constraints on the base monad or the prompt types; flavors of prompts and support for global prompts. The libraries are named
CCRef is closest to the interface of Dybvig, Peyton Jones and Sabry.
CCRef is derived from the definitional interpreter using the implementation techniques described and justified in the FLOPS 2010 paper. The monad transformer
CC implemented by
CCRef requires the base monad to support reference cells. In other words, the base monad must be a member of the type class
Mutable: that is, must be IO, ST, STM or their transformer.
CCRef adds to the original interface the frequently used function
abortP as a primitive.
As one may guess from their names, the libraries
CCCxe are closely related.
CCCxe is derived as a CPS version of
CCCxe is sometimes more efficient; it is always less perspicuous. Both libraries provide the identical interface and are interchangeable. It seems that
CCExc is faster at delimited control but imposes more overhead on the conventional code;
CCCxe is dual. It pays to use
CCCxe in code with long stretches of determinism punctuated by fits and restarts.
CCExc is the most direct implementation of the bubble-up reduction semantics of multi-prompt delimited control. The library stands out in not being based on the continuation monad. Rather, the monad of
CCExc is an extension of the Error monad: a monad for restartable exceptions. The library offers not one monad transformer but a family
(CC p) parameterized by the prompt flavor
p . The library defines several prompt flavors; the users are welcome to define their own.
Prompt flavors are inherently like exception flavors (the FLOPS 2010 paper even calls prompts `exception types' or `exception envelopes').
Control.Exception defines singular global exceptions such as
BlockedOnDeadMVar . There are global exceptions like
ErrorCall parameterized by the error string. There are closed global variants, such as
ArithException , with the fixed number of alternatives. There are also open variants,
SomeException , with any number of potential alternatives. Users may define their own exception types, whose visibility may be restricted to a module or a package. Finally, one may even generate distinct expression types dynamically, although that is seldom needed.
CCCxe support all these flavors. On one end is the prompt flavor
PS w . There is only one prompt of that flavor,
ps , which is globally defined and does not have to be passed around. The monad transformer
(CC (PS w)) then is the monad transformer for regular, single-prompt delimited continuations, for the answer-type
w . The Danvy/Filinski test, which looks in Scheme as
(display (+ 10 (reset (+ 2 (shift k (+ 100 (k (k 3))))))))appears as follows in Haskell:
test5 = (print =<<) . runCC $ incr 10 . pushPrompt ps $ incr 2 . shiftP ps $ \sk -> incr 100 $ sk =<< (sk 3)where
incr :: Monad m => Int -> m Int -> m Int incr n = ((return . (n +)) =<<)
One should read the operator
(=<<) , the flipped bind, as a ``call-by-value application'', akin to the application in call-by-value languages like Scheme. The application
f =<< e first evaluates the argument
e , performing its effects. The result is passed to
f , which is evaluated in turn. The application
sk 3 is an optimized version of
sk =<< (return 3) .
The appearance of
test5 is the IO computation. If we rather had the result of
test5 as a pure value (an integer), we merely need to apply
runST to the the
The sample code file
Generator1.hs shows one example of
PS; the file
SRReifT.hs of the LogicT library is a larger example. The sample code file
Generator2.hs demonstrates why we may need several prompts, perhaps with different types.
CCExc offers several flavors of multiple prompts: closed unions
P2 and open unions
PD . The open unions are like
SomeException . The prompt flavor
PD carries an extra integer identifier to further distinguish prompts of the same type. We may therefore dynamically generate an arbitrary number of
PD prompts, which was required in Dybvig, Peyton Jones and Sabry's framework.
The implementations of the three libraries
Basic interface for reference cell, used by
Regression test suite, with many examples of different flavors of prompts
A micro-benchmark for estimating the overhead of exercising delimited control. The benchmark does not help in measuring the indirect overhead, imposed by the libraries on the code that that uses no delimited control operators. Therefore, the benchmark is not realistic.
More interesting sample code, implementing generators like those of Python. The second file tells why one prompt is not enough.
PDF page 57 defines the bubble-up (bottom-up) reduction semantics of multi-prompt delimited control, which is implemented by
CCExc in the most straightforward way.
LogicT - backtracking monad transformer with fair operations and pruning
That library has been ported to the new implementation of delimited control,
CCCxe . One of the sample applications, of a computer playing 5x5 tic-tac-toe against itself, was used as a macro-benchmark of the new libraries. The end of the file TicTacToe.hs summarizes the results. The new libraries are faster.
The particularly elegant application of the calculus is typed sprintf:
sprintf format_expression arg1 arg2 ... . The type system ensures that the number and the types of format specifiers in
format_expression agree with the number and the types of
arg1 , etc. With control operators supporting answer-type modification and polymorphism,
sprintf is expressible as a regular, simple function.
The Haskell98 implementation below is the first implementation of delimited continuations with answer-type modification, polymorphism, effect typing and type inference in a widely available language. Thanks to parameterized (generalized) monads the implementation is the straightforward translation of the rules of the calculus. Less than a month later Matthieu Sozeau has defined generalized monad typeclass in the recent version of Coq and so implemented the calculus along with the type-safe sprintf in Coq.
Genuine shift/reset in Haskell98
Announcement of the Haskell implementation and the explanation of the implementation in terms of parameterized (generalized) monads. It has been posted on the Haskell mailing list on Wed, 12 Dec 2007 02:23:52 -0800 (PST).
The complete source code and the test, including the type-safe sprintf
Type-safe Formatted IO
Many more implementations of type-safe sprintf and sscanf
Variable (type)state `monad'
Description of the parameterized (generalized) monads
Matthieu Sozeau: The Proved Program of The Month - Type-safe printf via delimited continuations
< http://www.lri.fr/perso/~sozeau/repos/coq/misc/shiftreset/GenuineShiftReset.html >
``In this development we define the generalized monad typeclass and one particular instance: the continuation monad with variable input and output types. The latter lets us define shift and reset delimited control operators with effect typing, answer-type modification, and polymorphism. As an application of these operators, we build a type-safe sprintf.''
The two distinguished features of OchaCaml are the type system for
reset with the answer-type
modification and polymorphism, and the direct style of its control
operators. The first feature permits the examples of delimited
control that are not possible with the
Direct-style lets us write expressions with nested applications,
typical of applicative programming. We attempt to emulate both
features in Haskell, so to write OchaCaml examples similarly to
the way they are written in OchaCaml.
We rely on
RebindableSyntax to hide parameterized monad plumbing as
much as possible.
shift). This R5RS Scheme code is parameterized by two boolean flags:
keep-delimiter-upon-effect. The four combinations of the two flags correspond to the four delimited control operators. We can change the flags at run-time and so mutate
controlat run-time. The regression test code does exactly that, so it can test all four
The code relies on
call/cc for capturing undelimited continuations, and uses one global mutable cell. This turns out sufficient for implementing not only
shift/reset (Danvy/Filinski) but also for
control/prompt and the other
F operators. In contrast to Sitaram/Felleisen's implementation of
control , our code needs no equality on continuations. Our code is also far simpler. Our implementation of all four
F operators is direct rather than an emulation, and hence has the best performance.
This implementation immediately leads to a CPS transform for
prompt, thus confirming the result by Chung-chieh Shan. That
transform turns almost identical to the one discussed in the
Dybvig, Peyton Jones and Sabry's paper.
Multi-prompt delimited control in R5RS Scheme
R. Kent Dybvig, Simon Peyton Jones, and Amr Sabry: A Monadic Framework for Delimited Continuations.
JFP, v17, N6, pp. 687--730, 2007.
< http://www.cs.indiana.edu/cgi-bin/techreports/TRNNN.cgi?trnum=TR615 >
cuptoto multi-prompt versions of
shift-- providing the superset of the interface proposed by Dybvig, Peyton Jones and Sabry.
The library relies on
call/cc for capturing continuations. As any other
implementation of delimited control in terms of
the original Filinski's implementation of shift, the library must
be used with extreme care in the presence of other effects:
dynamic binding, exceptions, or independent uses of
The Scheme implementation of delimcc attests to the generality of the scAPI-based approach to implementing delimited control. The presented code is a straightforward translation of the
ccRef Haskell code. Although the present code works on any R5RS Scheme system, good performance should be expected only on the systems that implement
call/cc efficiently, such as Chez Scheme, Scheme48, Gambit, Larceny.
By specializing the code to the single prompt and to the
shift operator, we obtain a new implementation of
in R5RS Scheme. Even on systems that support
only inefficiently, this implementation has an advantage of not
leaking memory. The captured continuation, reified by
is just the needed prefix of the full continuation,
call/cc copies the whole stack. In other words,
we obtain a so-called JAR hack (see shift-reset.scm in Scheme48
The test suite
Ordinary shift/reset in R5RS Scheme
This implementation is derived by specializing the multi-prompt implementation to the single prompt and to the
shift operator. The code includes a few regression tests and two memory-leak tests.
A micro-benchmark, due to Gasbichler and Sperber
dynamic-wind. Fortunately, delimited control operators let application programmers write
dynamic-windthemselves; that function is no longer a primitive, is no longer hard-to-explain, and no longer has to be provided by the implementation. We show a sample code, as a generalization of the familiar re-throwing of exceptions.
dynamic-wind is one of the most complex Scheme
procedures. The mere size of its description in R5RS or the draft
R7RS, let alone time to understand it, is telling. And yet
the procedure is indispensable to prevent leaking of resources. Consider
the code that uses
call/cc for a non-local exit from processing file
(call/cc (lambda (exit) (with-input-from-file "file-name" (lambda () (let ((x (read))) (if (some-test x) (exit #f)) (process x))))))The Scheme procedure
with-input-from-filetakes care of opening the file, and closing it upon return. Alas, if the non-local exit is taken the file will remain open.
The problems are more serious than the mere failure to close the file. Here is an example of a non-local transfer of control silently breaking the implementation of dynamic binding. Suppose we are writing a pretty-printer and introduce a dynamically bound parameter for the target line width. We implement this parameter using the efficient technique of so-called ``shallow binding''.
(define current-line-width 80) (define (with-new-line-width new-lw thunk) (let* ((old current-line-width) (_ (set! current-line-width new-lw)) (r (thunk)) (_ (set! current-line-width old))) r)) (define (task title) (display title) (display "Current line width: ") (display current-line-width) (newline)) (define (ex2 flag) (task "Begin. ") (call/cc (lambda (exit) (with-new-line-width 120 (lambda () (task "Inner1. ") (if flag (exit #f)) (task "Inner2. "))))) (task "End. "))The task "Inner" is executed in its own dynamic environment, when
current-line-widthis bound to 120 from the default 80. Whereas the transcript of running
(ex2 #f)shows that
current-line-widthis restored at the end, for
current-line-widthis still 120 at the end. The non-local transfer of control broke the implementation.
We must use
dynamic-wind when re-binding the dynamic variable, so to
restore the old binding on normal or `abnormal' exit.
(define (with-new-line-width-dw new-lw thunk) (let ((old #f)) (dynamic-wind (lambda () ; before-thunk (set! old current-line-width) (set! current-line-width new-lw)) thunk ; real work (lambda () ; after-thunk (set! current-line-width old)) )))The code also restores the new binding when the dynamic scope is re-entered through the captured continuation -- see the example in the accompanying code.
Obviously the same problems occur if we use delimited continuation
operators rather than
call/cc for non-local transfer of
control. Delimited control also needs something like
Fortunately, delimited control lets us write
shift, we shall use a wholly equivalent
yield. Many uses of
shift are actually
shift can be written in terms of
yield, and vice versa,
no expressivity is lost.)
(define (yield-record-tag) yield-record-tag) (define (make-yield-record v k) (list yield-record-tag v k)) (define-syntax try-yield (syntax-rules () ((try-yield exp (r on-r) (v k on-y)) (let ((exp-r exp)) (if (and (pair? exp-r) (eq? (car exp-r) yield-record-tag)) (let ((v (cadr exp-r)) (k (caddr exp-r))) on-y) (let ((r exp-r)) on-r)))))) (define (yield v) (shift k (make-yield-record v k)))
Here is the implementation of
dynamic-wind, with the standard
(define (dyn-wind-yield before-thunk thunk after-thunk) (let loop ((th (lambda () (reset (thunk))))) (before-thunk) (let ((res (th))) (after-thunk) (try-yield res (r r) ; return the result (v k (let ((reenter (yield v))) (loop (lambda () (k reenter))))) ))))It is the drop-in replacement for R5RS
call/cccan also be written in terms of
yield. The accompanying code demonstrates how old examples of
dynamic-windcontinue to work when both are re-implemented in terms of
The last example in the accompanying code shows a simple generator, or two coroutines, each with its own dynamic environment:
(define (ex7) (task "Begin. ") (for-loop (lambda () ; generator (with-new-line-width-yl 120 (lambda () (task "Inner1. ") (yield 1) (task "Inner2. ") (yield 2) (task "Inner3. ")))) (lambda (v) ; loop body (display "Yielded value: ") (display v) (newline) (task "Loop body. "))) (task "End. "))We see from the transcript that switching coroutines also switches the binding to
current-line-width, in and out, several times.
We have demonstrated that delimited control operators,
yield in particular,
let us implement `finally'-like constructs such as
The implementation is quite like the standard implementation of
in terms of exception-handling primitives, keeping in mind that
yield is akin to a (multiply) restartable exception.
The dynamic-wind problem
Your comments, problem reports, questions are very welcome!
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