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continuation (n.)
1.the act of continuing an activity without interruption
2.the consequence of being lengthened in duration
3.a Gestalt principle of organization holding that there is an innate tendency to perceive a line as continuing its established direction
4.a part added to a book or play that continues and extends it
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Merriam Webster
ContinuationCon*tin`u*a"tion (?), n. [L. continuatio: cf. F. connuation.]
1. That act or state of continuing; the state of being continued; uninterrupted extension or succession; prolongation; propagation.
Preventing the continuation of the royal line. Macaulay.
2. That which extends, increases, supplements, or carries on; as, the continuation of a story.
My continuation of the version of Statius. Pope.
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⇨ definição - Wikipedia
continuation (n.)
addition, circle, consolidation, continuance, epilogue, eternity, extension, good continuation, instalment, law of continuation, lengthiness, permanence, perpetuation, postscript, procession, progression, prolongation, protraction, sequel, supplement, survival
Ver também
continuation (n.)
↘ closing, closing down, discontinuance, discontinuation ↗ carry on, carry on with, continue, continue with, go on, go on with, keep .., keep on .., persist, persist in, persist with, proceed with, pursue
⇨ Analytic continuation • Call-with-current-continuation • Continuation (album) • Continuation (disambiguation) • Continuation War • Continuation car • Continuation high school • Continuation high schools • Continuation map • Continuation-passing style • Delimited continuation • Finnish 3rd Division (Continuation War) • Finnish 5th Division (Continuation War) • Finnish 6th Division (Continuation War) • Finnish 7th Division (Continuation War) • Finnish III Corps (Continuation War) • Line continuation • Line continuation character • List of Finnish divisions in the Continuation War • List of units of the Finnish Air Force during the Continuation War • Melbourne Continuation School • New Valley Continuation High School • Spectrum continuation analysis • Tracy High Continuation School • Upward continuation
continuation (n.)
continuity; duration; continuance; continuation[ClasseHyper.]
action, activity, busyness, employment, occupation, pursuit[Hyper.]
carry on, continue, go on, proceed - continue, go forward, proceed - carry on, continue, get about, go along, go on, keep, keep going, keep it up, keep on, proceed - continue - continue, persist in[Dérivé]
closing, closing down, discontinuance, discontinuation[Ant.]
continuation (n.)
duration, length - delay, hold, postponement, time lag, wait[Hyper.]
continue - drag out, draw out, extend, prolong, protract, spin out, stretch out - carry on, continue, go on, proceed - continue, keep, keep going, keep on, retain - keep up, prolong, sustain - carry on, continue, get about, go along, go on, keep, keep going, keep it up, keep on, proceed - be left, be left over, continue, remain, stay, stay on - continue - drawn-out, extended, lengthy, prolonged, protracted[Dérivé]
continuation (n.)
continuation (n.)
feuilleton (fr)[DomainDescrip.]
continuation (nf.)
continuity; duration; continuance; continuation[Classe]
action de (ou fait d'être) (fr)[Classe...]
action répétée (fr)[ClasseParExt.]
action de continuer après une interruption (fr)[ClasseHyper.]
partie (fr)[Classe...]
continuance, continuation[Hyper.]
pick up, restart, re-start, resume, take up[Nominalisation]
Wikipedia
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In computer science and programming, a continuation is an abstract representation of the control state of a computer program. A continuation reifies the program control state, i.e. the continuation is a data structure that represents the computational process at a given point in the process' execution; the created data structure can be accessed by the programming language, instead of being hidden in the runtime environment. Continuations are useful for encoding other control mechanisms in programming languages such as exceptions, generators, coroutines, and so on.
The "current continuation" or "continuation of the computation step" is the continuation that, from the perspective of running code, would be derived from the current point in a program's execution. The term continuations can also be used to refer to first-class continuations, which are constructs that give a programming language the ability to save the execution state at any point and return to that point at a later point in the program.
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The earliest description of continuations was made by Adriaan van Wijngaarden in September 1964. Wijngaarden spoke at the IFIP Working Conference on Formal Language Description Languages held in Baden bei Wien, Austria. As part of a formulation for an Algol 60 preprocessor, he called for a transformation of proper procedures into continuation-passing style.[1]
Christopher Strachey, Christopher P. Wadsworth and John C. Reynolds brought the term continuation into prominence in their work in the field of denotational semantics that makes extensive use of continuations to allow sequential programs to be analysed in terms of functional programming semantics.[1]
Steve Russell invented the continuation in his second Lisp implementation for the IBM 704, though he did not name it.[2] However there is no cross references about that and the given reference is a mere claim.
A complete history of the discovery of continuations is given by (Reynolds 1993).
First-class continuations are a language's ability to completely control the execution order of instructions. They can be used to jump to a function that produced the call to the current function, or to a function that has previously exited. One can think of a first-class continuation as saving the state of the program. However, it is important to note that true first-class continuations do not save program data, only the execution context. This is illustrated by the "continuation sandwich" description:
Say you're in the kitchen in front of the refrigerator, thinking about a sandwich. You take a continuation right there and stick it in your pocket. Then you get some turkey and bread out of the refrigerator and make yourself a sandwich, which is now sitting on the counter. You invoke the continuation in your pocket, and you find yourself standing in front of the refrigerator again, thinking about a sandwich. But fortunately, there's a sandwich on the counter, and all the materials used to make it are gone. So you eat it. :-)[3]
Scheme was the first full production system, providing first "catch"[1] and then call/cc. Bruce Duba introduced call/cc into SML.
Continuations are also used in models of computation including denotational semantics, the Actor model, process calculi, and lambda calculus. These models rely on programmers or semantics engineers to write mathematical functions in the so-called continuation-passing style. This means that each function consumes a function that represents the rest of the computation relative to this function call. To return a value, the function calls this "continuation function" with a return value; to abort the computation it returns a value.
Functional programmers who write their programs in continuation-passing style gain the expressive power to manipulate the flow of control in arbitrary ways. The cost is that they must maintain the invariants of control and continuations by hand, which is a highly complex undertaking.
Continuations are useful because they simplify and clarify the implementation of several other common programming design patterns, including coroutines (aka green threads), and exception handling. Continuations provide the basic, low-level primitive unifying these otherwise seemingly unconnected patterns. Continuations can provide elegant solutions to some difficult high-level problems, in particular the problem of programming a web server that supports multiple pages, accessed by the user's use of the forward and back buttons and by following links. The Seaside web framework in Smalltalk uses continuations to great effect, allowing one to program the web server in procedural style, by switching continuations when switching pages.
More complex constructs exist as well, for which "continuations provide an elegant description".[1] For example, in C, setjmp can be used to jump from the middle of one function to another function, provided the second function lies deeper in the stack (if it is waiting for the first function to return, possibly among others). Other more complex examples include coroutines in Simula 67, Lua, and Perl; tasklets in Stackless Python; generators in Icon and Python; continuations in Scala (starting in 2.8); fibers in Ruby (starting in 1.9.1); the backtracking mechanism in Prolog; monads in functional programming; and threads.
The Scheme programming language includes the control operator call-with-current-continuation (abbreviated as: call/cc) with which a Scheme program can manipulate the flow of control:
(define the-continuation #f) (define (test) (let ((i 0)) ; call/cc calls its first function argument, passing ; a continuation variable representing this point in ; the program as the argument to that function. ; ; In this case, the function argument assigns that ; continuation to the variable the-continuation. ; (call/cc (lambda (k) (set! the-continuation k))) ; ; The next time the-continuation is called, we start here. (set! i (+ i 1)) i))
Defines a function test
that sets the-continuation
to the future execution state of itself:
> (test)
1
> (the-continuation)
2
> (the-continuation)
3
> ; stores the current continuation (which will print 4 next) away
> (define another-continuation the-continuation)
> (test) ; resets the-continuation
1
> (the-continuation)
2
> (another-continuation) ; uses the previously stored continuation
4
For a gentler introduction to this mechanism, see call-with-current-continuation.
This example shows a possible usage of continuations to implement coroutines as separate threads.[4]
;;; A naive queue for thread scheduling. ;;; It holds a list of continuations "waiting to run". (define *queue* '()) (define (empty-queue?) (null? *queue*)) (define (enqueue x) (set! *queue* (append *queue* (list x)))) (define (dequeue) (let ((x (car *queue*))) (set! *queue* (cdr *queue*)) x)) ;;; This starts a new thread running (proc). (define (fork proc) (call/cc (lambda (k) (enqueue k) (proc)))) ;;; This yields the processor to another thread, if there is one. (define (yield) (call/cc (lambda (k) (enqueue k) ((dequeue))))) ;;; This terminates the current thread, or the entire program ;;; if there are no other threads left. (define (thread-exit) (if (empty-queue?) (exit) ((dequeue))))
The functions defined above allow for defining and executing threads through cooperative multitasking, i.e. threads that yield control to the next one in a queue:
;;; The body of some typical Scheme thread that does stuff: (define (do-stuff-n-print str) (lambda () (let loop ((n 0)) (format #t "~A ~A\n" str n) (yield) (loop (1+ n))))) ;;; Create two threads, and start them running. (fork (do-stuff-n-print "This is AAA")) (fork (do-stuff-n-print "Hello from BBB")) (thread-exit)
The previous code will produce this exit:
This is AAA 0 Hello from BBB 0 This is AAA 1 Hello from BBB 1 This is AAA 2 Hello from BBB 2 ...
A program must allocate space in memory for the variables its functions use. Most programming languages use a call stack for storing the variables needed because it allows for fast and simple allocating and automatic deallocation of memory. Other programming languages use a heap for this, which allows for flexibility at a higher cost for allocating and deallocating memory. Both of these implementations have benefits and drawbacks in the context of continuations.[5]
Many programming languages exhibit first-class continuations under various names; specifically:
setcontext
et al. (POSIX)async
and await
: “sign up the rest of method as the continuation, and then return to your caller immediately; the task will invoke the continuation when it completes.” Asynchronous Programming for C#Continuation
callcc0
and callcc1
Control.Monad.Cont
create, suspend, @
operator: coexpressionsContinuation
PMC; uses continuation-passing style for all control flowcall(exp())
and continue(aContinuation, anyValue)
callcc
Responder
call-with-current-continuation
(commonly shortened to call/cc
)Continuation currentDo:
, in most modern Smalltalk environments continuations can be implemented without additional VM support.SMLofNJ.Cont.callcc
c
, the flow control operation for call with current continuationIn any language which supports closures and proper tail calls, it is possible to write programs in continuation-passing style and manually implement call/cc. (In continuation-passing style, call/cc becomes a simple function that can be written with lambda.) This is a particularly common strategy in Haskell, where it is easy to construct a "continuation-passing monad" (for example, the Cont
monad and ContT
monad transformer in the mtl
library). The support for proper tail calls is needed because in continuation-passing style no function ever returns; all calls are tail calls.
One area that has seen practical use of continuations is in Web programming.[6][7] The use of continuations shields the programmer from the stateless nature of the HTTP protocol. In the traditional model of web programming, the lack of state is reflected in the program's structure, leading to code constructed around a model that lends itself very poorly to expressing computational problems. Thus continuations enable code that has the useful properties associated with inversion of control, while avoiding its problems. Inverting back the inversion is a paper that provides a good introduction to continuations applied to web programming.
Some of the more popular continuation-aware Web servers are the Racket Web Server, the UnCommon Web Framework and Weblocks Web framework for Common Lisp, the Seaside framework for Smalltalk, Ocsigen/Eliom for OCaml, Continuity for Perl, Wee for Ruby, and the Nagare framework for Python. The Apache Cocoon Web application framework also provides continuations (see the Cocoon manual).
Support for continuations varies widely. A programming language supports re-invocable continuations if a continuation may be invoked repeatedly (even after it has already returned). Re-invocable continuations were introduced by Peter J. Landin using his J (for Jump) operator that could transfer the flow of control back into the middle of a procedure invocation. Re-invocable continuations have also been called "re-entrant" in the Racket language. However this use of the term "re-entrant" can be easily confused with its use in discussions of multithreading.
A more limited kind is the escape continuation that may be used to escape the current context to a surrounding one. Many languages which do not explicitly support continuations support exception handling, which is equivalent to escape continuations and can be used for the same purposes. C's setjmp/longjmp
are also equivalent: they can only be used to unwind the stack. Escape continuations can also be used to implement tail call elimination.
One generalization of continuations are delimited continuations. Continuation operators like call/cc
capture the entire remaining computation at a given point in the program and provide no way of delimiting this capture. Delimited continuation operators address this by providing two separate control mechanisms: a prompt that delimits a continuation operation and a reification operator such as shift
or control
. Continuations captured using delimited operators thus only represent a slice of the program context.
Continuations are the functional expression of the GOTO statement, and the same caveats apply.[8] While they are a sensible option in some special cases such as web programming, use of continuations can result in code that is difficult to follow. In fact, the esoteric programming language Unlambda includes call-with-current-continuation as one of its features solely because of its resistance to understanding[citation needed]. The external links below illustrate the concept in more detail.
Some phenomena in natural languages have been described using the notion of delimited continuation.
Informally speaking, continuations can account for the similarity between such sentences as "Alice sees Bob"—formally, —and "Alice sees everyone", i.e. .
See Chris Barker's paper Continuations in Natural Language for details. See also Montague grammar.
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