Special thanks to many of you for your very decent replies. On Oct 9, 11:18 am, George Neuner <gneuner2/@/comcast.net> wrote: > On Tue, 09 Oct 2007 05:15:49 -0000, [EMAIL PROTECTED] wrote: > >Again I am depressed to encounter a fundamentally new concept that I > >was all along unheard of. Its not even in paul graham's book where i > >learnt part of Lisp. Its in Marc Feeley's video. > > >Can anyone explain: > > >(1) its origin > > Lambda calculus. Continuation is just a formal term for "what the > code does next". It manifests, literally, as the next instruction(s) > to be executed. > > >(2) its syntax and semantics in emacs lisp, common lisp, scheme > > Lisp does not have explicit continuations so there is no syntax for > them. Continuations in Lisp mainly take the form of function calls, > function returns, exceptions, conditions, etc. Sometimes code is > written in "continuation passing style" (CPS) in which each function > has one or more additional function parameters (the continuations) - > the function terminates by passing its result as an argument to one of > those continuation functions. > > Scheme has explicit continuations based on closures. Closure > continuations are created using CALL-WITH-CURRENT-CONTINUATION > (usually abbreviated as CALL/CC). Some Schemes also recognize a > LET/CC form used mainly for escape continuations (exceptions). > Scheme's closure continuations can be stored in data structures and > used for complex control forms such as multitasking. Like Lisp, > Scheme code also is sometimes written using CPS. > > >(3) Is it present in python and java ? > > It is present in all languages. It generally takes the form of > procedure or function calls, returns, exceptions, etc. > > >(4) Its implementation in assembly. for example in the manner that > >pointer fundamentally arises from indirect addressing and nothing new. > >So how do you juggle PC to do it. > > As I stated above, every function call or return _is_ a continuation > ... their implementation is obvious. > > For the closure form used in Scheme, the implementation is to create a > closure, a data structure containing the function address and some > method of accessing the function's free variables, and to call the > function. How you do this depends greatly on the instruction set. > > >(5) how does it compare to and superior to a function or subroutine > >call. how does it differ. > > Calling closure continuations is a little more complicated and a bit > slower than calling a normal function. Creating the closure in the > first place may be simple or complicated depending on the complexity > of the source code and the processor's instruction set. > > >Thanks a lot. > > >(6) any good readable references that explain it lucidly ? > > Get yourself a good textbook on compilers. Most of the techniques are > applicable to all languages - even for seemingly very different > languages, the differences in their compilers are simply in how the > basic compilation techniques are combined. > > My favorite intermediate-level books are > > Aho, Sethi & Ullman. "Compilers: Principles, Techniques and Tools". > 2nd Ed. 2006. ISBN 0-321-48681-1. > The first edition from 1986, ISBN 0-201-10088-6, is also worth having > if you can still find it. The 1st edition is mainly about procedural > languages, the 2nd gives more time to functional languages and modern > runtime issues like GC and virtual machines. > > Cooper & Torczon, "Engineering a Compiler", 2004. > ISBN 1-55860-698-X (hardcover), 1-55860-699-8 (paperback). > Also available as a restricted 90-day ebook > fromhttp://rapidshare.com/files/24382311/155860698X.Morgan_20Kaufmann.Eng... > > There are also some decent intro books available online. They don't > go into excruciating detail but they do cover the basics of code > shaping which is what you are interested in. > > Torben Mogensen. "Basics of Compiler > Design"http://www.diku.dk/~torbenm/Basics/ > > "Engineering a Compiler". I don't have this author's name, nor can > Google find it at the moment. I have a copy though (~2MB) - if you > are interested, contact me by email and I'll send it to you. > > Also Google for free CS books. Many older books (including some > classics) that have gone out of print have been released > electronically for free download. > > George > -- > for email reply remove "/" from address
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