Continuation (nonfiction)

In computer science and computer programming, a continuation is an abstract representation of the control state of a computer program. A continuation implements (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's 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, subroutines, 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, possibly multiple times.

In Web development

One area that has seen practical use of continuations is in Web programming. 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 of control or, Continuations versus page-centric programming 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, Tales Framework for Fantom and the Nagare framework for Python, Wt for C++, MFlow for Haskell. The Apache Cocoon Web application framework also provides continuations (see the Cocoon manual).

Disadvantages

Continuations are the functional expression of the GOTO statement, and the same caveats apply. 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.

See also

• Abstraction (computer science) (nonfiction) - In software engineering and computer science, abstraction is:
  • the process of removing physical, spatial, or temporal details or attributes in the study of objects or systems in order to focus attention on details of higher importance, it is also very similar in nature to the process of generalization;
  • the creation of abstract concept-objects which are created by mirroring common features or attributes from various non-abstract objects or systems of study — the result of the process of abstraction.
• Computer program (nonfiction) - a collection of instructions that performs a specific task when executed by a computer.
• Computer programming (nonfiction)
• Computer science (nonfiction)
• Control flow (nonfiction) - the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an imperative programming language from a declarative programming language. Within an imperative programming language, a control flow statement is a statement, the execution of which results in a choice being made as to which of two or more paths to follow. For non-strict functional languages, functions and language constructs exist to achieve the same result, but they are usually not termed control flow statements.
• Exception handling (nonfiction) - the process of responding to the occurrence, during computation, of exceptions – anomalous or exceptional conditions requiring special processing – often disrupting the normal flow of program execution. It is provided by specialized programming language constructs, computer hardware mechanisms like interrupts or operating system IPC facilities like signals. In general, an exception breaks the normal flow of execution and executes a pre-registered exception handler. The details of how this is done depends on whether it is a hardware or software exception and how the software exception is implemented. Some exceptions, especially hardware ones, may be handled so gracefully that execution can resume where it was interrupted. Alternative approaches to exception handling in software are error checking, which maintains normal program flow with later explicit checks for contingencies reported using special return values or some auxiliary global variable such as C's errno or floating point status flags; or input validation to preemptively filter exceptional cases.
• Generator (computer programming) (nonfiction) - a routine that can be used to control the iteration behavior of a loop.
• Goto (nonfiction) -
• Iteration (nonfiction) - the repetition of a process in order to generate a (possibly unbounded) sequence of outcomes. The sequence will approach some end point or end value. Each repetition of the process is a single iteration, and the outcome of each iteration is then the starting point of the next iteration. In mathematics and computer science, iteration (along with the related technique of recursion) is a standard element of algorithms.
• Loop (computer programming) (nonfiction) - a sequence of statements which is specified once but which may be carried out several times in succession. The code "inside" the loop (the body of the loop) is obeyed a specified number of times, or once for each of a collection of items, or until some condition is met, or indefinitely.
• Reification (computer science) (nonfiction) - the process by which an abstract idea about a computer program is turned into an explicit data model or other object created in a programming language. A computable/addressable object—a resource—is created in a system as a proxy for a non computable/addressable object. By means of reification, something that was previously implicit, unexpressed, and possibly inexpressible is explicitly formulated and made available to conceptual (logical or computational) manipulation. Informally, reification is often referred to as "making something a first-class citizen" within the scope of a particular system. Some aspect of a system can be reified at language design time, which is related to reflection in programming languages. It can be applied as a stepwise refinement at system design time. Reification is one of the most frequently used techniques of conceptual analysis and knowledge representation.
• Runtime system (nonfiction) - also called run-time system, runtime environment or run-time environment, primarily implements portions of an execution model. This is not to be confused with the runtime lifecycle phase of a program, during which the runtime system is in operation. Most languages have some form of runtime system that provides an environment in which programs run. This environment may address a number of issues including the layout of application memory, how the program accesses variables, mechanisms for passing parameters between procedures, interfacing with the operating system, and otherwise. The compiler makes assumptions depending on the specific runtime system to generate correct code. Typically the runtime system will have some responsibility for setting up and managing the stack and heap, and may include features such as garbage collection, threads or other dynamic features built into the language.
• Subroutine (nonfiction) - a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed. Subroutines may be defined within programs, or separately in libraries that can be used by many programs. In different programming languages, a subroutine may be called a procedure, a function, a routine, a method, or a subprogram. The generic term callable unit is sometimes used. The name subprogram suggests a subroutine behaves in much the same way as a computer program that is used as one step in a larger program or another subprogram. A subroutine is often coded so that it can be started several times and from several places during one execution of the program, including from other subroutines, and then branch back (return) to the next instruction after the call, once the subroutine's task is done.

Fiction cross-reference

• Crimes against mathematical constants
• Gnomon algorithm
• Gnomon Chronicles

External links

• Continuation @ Wikipedia