Jensen's Device - Description

Description

Jensen's device exploits call by name and side-effects. Call by name is an argument passing convention that delays the evaluation of an argument until it is actually used in the procedure (a consequence of the copy rule for procedures). Algol introduced call by name.

A classic example of Jensen's device is a procedure that computes the sum of a series, :

real procedure Sum(k, l, u, ak) value l, u; integer k, l, u; real ak; comment k and ak are passed by name; begin real s; s := 0; for k := l step 1 until u do s := s + ak; Sum := s end;

In the procedure, the index variable k and summation term ak are passed by name. Call by name enables the procedure to change the value of the index variable during execution of the for loop. Call by name also causes the ak argument to be reevaluated during each iteration of the loop. Typically, ak will depend upon the changing (side-effected) k.

For example, code to compute the first 100 terms of a real array V would be:

Sum(i, 1, 100, V).

During the execution of Sum, the actual argument i will increment during each step of the for loop, and each of the procedure's evaluations of ak will use the current value of i to access the successive array elements V.

Jensen's device is general. A double summation can be done as:

Sum(i, l, m, Sum(j, l, n, A))

The Sum function can be employed for arbitrary functions merely by employing the appropriate expressions. If a sum of integers were desired the expression would be just Sum(i,1,100,i);, if a sum of squares of integers, then Sum(i,1,100,i*i);, and so on. A slight variation would be suitable for initiating a numerical integration of an expression by a method very similar to that of Sum.

The evaluation of ak is implemented with a thunk, which is essentially a subroutine with an environment. The thunk is a closure with no arguments. Each time a procedure needs the value of its formal argument, it simply calls the thunk. The thunk evaluates the actual argument in the scope of the calling code (not the scope of the procedure).

In the absence of this pass-by-name facility, it would be necessary to define functions embodying those expressions to be passed according to the protocols of the computer language, or to create a compendium function along with some arrangement to select the desired expression for each usage.

Read more about this topic:  Jensen's Device

Other articles related to "description, descriptions":

History of Knowledge of Ventricular Fibrillation
... Whether this is a description of ventricular fibrillation is debatable ... The next recorded description occurs 3000 years later and is recorded by Vesalius, who described the appearance of "worm-like" movements of the heart in animals prior to death ... The significance and clinical importance of these observations and descriptions possibly of ventricular fibrillation were not recognised until John Erichsen in 1842 described ...
Meta Element Used in Search Engine Optimization - The description Attribute
... Unlike the keywords attribute, the description attribute is supported by most major search engines, like Yahoo! and Bing, while Google will fall back on this tag when information about the page itself is ... The description attribute provides a concise explanation of a Web page's content ... the Web page authors to give a more meaningful description for listings than might be displayed if the search engine was unable to automatically create its own ...
Gerald Of Wales - Natural History
... He gives a vivid and accurate description of the last colony of the European Beaver in Wales on the River Teifi, but spoils it by repeating the legend that beavers castrate themselves to avoid danger ... Likewise he gives a good description of an Osprey fishing, but adds the mythical detail that the bird has one webbed foot ... His description of Irish wildlife was harshly called "worthless" the better view perhaps is that despite its faults it gives a valuable glimpse of Irish fauna in the 1180s ...
Essay - Forms and Styles - Descriptive
... using descriptive language, and organizing the description are the rhetorical choices to be considered when using a description ... A description is usually arranged spatially but can also be chronological or emphatic ... The focus of a description is the scene ...
Universal Description Discovery And Integration
... Universal Description, Discovery and Integration (UDDI, pronounced Yu-diː) is a platform-independent, Extensible Markup Language (XML)-based registry by which businesses worldwide can list themselves on the ... and to provide access to Web Services Description Language (WSDL) documents describing the protocol bindings and message formats required to interact with the web services listed in its directory ...

Famous quotes containing the word description:

    A sound mind in a sound body, is a short, but full description of a happy state in this World: he that has these two, has little more to wish for; and he that wants either of them, will be little the better for anything else.
    John Locke (1632–1704)

    As they are not seen on their way down the streams, it is thought by fishermen that they never return, but waste away and die, clinging to rocks and stumps of trees for an indefinite period; a tragic feature in the scenery of the river bottoms worthy to be remembered with Shakespeare’s description of the sea-floor.
    Henry David Thoreau (1817–1862)

    Do not require a description of the countries towards which you sail. The description does not describe them to you, and to- morrow you arrive there, and know them by inhabiting them.
    Ralph Waldo Emerson (1803–1882)