Subject: Re: DSSSL side effect-freeness From: Paul Prescod <papresco@xxxxxxxxxxxxxxxx> Date: Tue, 27 Jan 1998 18:43:47 -0500 |
Frank A. Christoph wrote: > > I know we have been over this issue before, but would somebody please > reiterate the reason that the DSSSL expression language must be side > effect-free? It seems very odd to me that a language which has been so > conscientiously purged of side effects is nevertheless call-by-value. Do you really mean call-by-value and not "eager" vs. "lazy?" Call-by-value and call-by-reference are identical in the absence of mutability. > I > have mentioned before that, although DSSSL is side effect-free, it is not > "read effect-free", since there are procedures whose result depends on the > context, e.g., current-node, process-children. Is the CBV behavior > necessary in some way to allow lazy grove implementations? What about the > so-called side effect-freeness? Think about a grove that is 5GB large and distributed across many machines. Now you want to format a single paragraph. Under the DSSSL model, you must ask the other machines about the various parts of the grove, but you do *not* have to apply the entire stylesheet to the entire grove. At most you must apply the construction rules for the element and those of its ancestors. So you must build the whole grove (typically), but you need not apply the whole stylesheet to it. In a WYSIWYG system where you might want to look at a single page of a multi-gig document, that might make a big difference. Paul Prescod -- "You have the wrong number." "Eh? Isn't that the Odeon?" "No, this is the Great Theater of Life. Admission is free, but the taxation is mortal. You come when you can, and leave when you must. The show is continuous. Good-night." -- Robertson Davies, "The Cunning Man" DSSSList info and archive: http://www.mulberrytech.com/dsssl/dssslist
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