Subject: Re: DSSSL extensions for XSL From: christo@xxxxxxxxxxxxxxxxxx (Frank Christoph) Date: Mon, 29 Sep 1997 19:39:51 +0900 |
> > > An upwards-only continuation is one that can only be used to return to a > > > stack frame in the current call chain. When the call/cc call that > > > created a continuation has returned, the continuation can no longer be > > > called. > > > > Then that's call/ec. > > I've never come across that name. What Scheme systems provide a > procedure named call/ec? Hm, well I don't claim to be a Scheme expert and certainly not a historian. Perhaps my impression that it was a widely used name was mistaken. I know that Rice's MzScheme, which is the underlying interpreter in PLT's DrScheme package provides it. I tried checking a few other implementations like Chez Scheme and Gambit over the web, but couldn't get a hold of their library manuals online. The other implementation I have installed here, SCM, doesn't seem to provide it. > > I don't think it makes sense to call this "upwards-only" > > I didn't make this term up; I'm only using the terminlogy that I've come > across in the Scheme community. The Kawa docs use this term, and that's > certainly not the only place I've come across it. It makes a certain > amount of sense to me because the limitation on upwards-only > continuations is that they can only be used to return up the stack. I'm not saying you made it up. I did a little check of the literature. Luc Moreau uses "upwards" and "downwards" in the sense I used them in both "An Introduction to Continuations" and in his thesis, "Sound Evaluation of Parallel Functional Programs with First-Class Continuations". Kent Dybvig uses the terms "inwards" and "outwards" in his dissertation, "Three Implementation Models for Scheme". Daniel Friedman, Mitchell Wand and Christopher Haynes use "upwards" and "downwards" in the sense I used them in "Essentials of Programming Languages". I think that Matthias Felleisen and Robert Hieb use the terms in my sense in "The Revised Report on the Syntactic Theories of Sequential Control and State," (but I'm not sure). I didn't come across any papers or references that use the terms in the opposite sense. (I didn't check Kawa. I trust you. :) Well, whatever... it's a trivial point. The best thing is probably to distinguish between the two as "escaping" and "first-class" continuations. (Although, there is a problem even there because Reynolds originally introduced first-class continuations using the keyword "escape"!) --FC DSSSList info and archive: http://www.mulberrytech.com/dsssl/dssslist
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