On 6/13/02 7:49 AM, "Daniel Fagerström" <[EMAIL PROTECTED]> wrote:
> Diana Shannon wrote: > >> >> On Wednesday, June 12, 2002, at 06:31 PM, Daniel Fagerstrom wrote: >> >>> Feel free to ask if you need further clarification about my integration >>> proposal. >> >> >> What *I'm* most interested in is a statement you made a while ago on >> this list about the difficulties students can have learning about >> continuations, from your direct experience teaching continuations. > > > Why are continuations difficult? > -------------------------------- > > Continuations are a little bit like "goto on steroids". They are used in > the study of denotional semantics which is a subarea of theoretical > computer science. The aim of denotional semantics, is to describe (and > prove properties of) programming constructs in terms of a very small > functional language, (lambda calculus) that includes continuations. > Continuations are extremely powerfull; program language constructs like > assignement, goto, while, for-loops, throw-catch and coroutines to name > a few can be defined in terms of a terse and cryptic, (but elegant, if > you happen to like mathematical puzzles ;) functional program. > > Some functional program langages, Scheme e.g. have continuations. A good > use for continuations in such languages is to use them together with > macros to define new language constructions. You can also use them for > building a more intelectually demanding form of spagetti code ;) > > Why can continuation based code be difficult to grasp? Even if most of > us might not remember it anymore, learning to understand and use even > the simplest programming language construct required a considerable > effort once, allowing ad hoc introduction of new constructs forces us to > repeat this effort often. Continuations forces us to think about the > whole state of the program rather than about state changes. > Continuations introduces non-local effects in the code, non-local > effects are the main reason why gotos and exception handling can be hard > to follow. > > Flowmaps and continuations > -------------------------- > > In Flowmaps, continuations are used for implementing something quite > similar to coroutines. > >> Do you have any suggestions for the doc effort in this regard? > > > Focus on the coroutine aspect of the flowmaps, it is fairly easy to > understand, while general use of continuations just isn't. For the > flowmap user, the fact that flowmaps are implemented in terms of > continuations should be of very limited importance otherwise we have a > severe mix of concerns in the design. > > I strongly sugests that we should _not_ make general use of > continuations available in the flowmap language as there is no need for > it for what flowmaps are intended to do: describing multipage flow in > webapps. I definitely agree here. The last thing we want to do is describe continuations as the complex concepts from denotational semantics, or their usage patterns in Scheme. >> In what >> aspects do students have problems: understanding the concept, applying >> them to real-world situations, etc.? I've read up on continuations, >> generally, but so many existing docs are tied to Scheme implementations >> (which could frighten users, as has also been discussed). Can you point >> to any existing teaching resources that you use? Care to donate anything >> of your own, even a teaching outline? Any tips? > > > I tought denotatinal semantics, and the literature in that area is even > more frightening than the Scheme literature;) > > IMO we would make the effort to use flowmaps to high if we require > people to learn continuations. I belive that it should be enough to > teach about the coroutine like control structure, that happen to be > implemented in terms of continuations. If it's not possible I think that > is important feedback on that we have to work more on the basic concepts > in the flowmap. I have a long unfinished RT where I compare FSM:s with > the basics concepts in the Flowmap proposal and develop the coroutine > analog a bit further. I'll check if it is in a comprehensible state and, > if so, I'll send it to you. > > I would also like to remind about some mails I wrote a couple of months > ago where I explain why it's IMO is FS or even harmfull to allow local > variables in the flowmap language: see > http://marc.theaimsgroup.com/?l=xml-cocoon-dev&m=101861458122598&w=2 and > some other mails in the "continuation fear" thread. I need to write down the ideas which are specific to the usage of continuations in Cocoon's implementation. I've described most of these in various threads in the past, but I didn't manage to put them together in a single document. -- Ovidiu Predescu <[EMAIL PROTECTED]> http://www.geocities.com/SiliconValley/Monitor/7464/ (Apache, GNU, Emacs...) --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, email: [EMAIL PROTECTED]
