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Robustness (was Re: Expandable templates)

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Thu, 21 Oct 1999 21:01:30 +0200

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 David Carlisle wrote in reply to James Kilfiger:  > > but I get errors when I put instances in an \edef.  >  > The number of commands that work in an edef is vanishingly small;-)  > Probably (instances of) templates could be (or perhaps already are!)  > robust via the normal latex \protect mechanism, in which case they  > should survive the latex equivalent of \edef which is  > \[log in to unmask] instances of templates should be robust if they are not we should make them or at least their call through a standard mechanism like \UseInstance should be robust. this needs checking but is not significantly difficult to provide. more difficult might be to make a the use of \UseTemplate robust since that might contain arbitary code in its key/value segment.  > There is no way to make an instance of a template expandable in general  > as the key setting in the template call are typically low level register  > assignments in TeX, and they are not expandable. this answer that template instances can't be expandable (with reasonable effort --- disregarding that (i think) Alan Jeffrey once proved that one can implement everything in TeX's mouth if one is prepared to wait long enough for the answer (this is really the turing machine argument to please don't argue that machines get faster :-) but since the question of why does a command that is claimed to be "robust" fails in an edef and we had a similar "bug report (pr/3084)" just recently i appended my reply back then below since i think this is of some interest. questions:  a) anybody having any idea whatsoever to deal with this problem of robustness  other than LaTeX currently does? (conceptually i mean)  b) if the current scheme is basically sound (and i think it is) would be  trapping \edef a sensible approach, ie a plain edef would effectively run  \protected@edef and a code writer who really would like to use a vanilla  \edef for speed reasons would have to use something like \unprotected@edef?  This would clearly help in cases like 3084 or in James' usage. but it would  slow down processing a lot if packages and the latex kernel are not adjusted  accordingly (the kernel currently contains 175 \[xe]def's in places where we  thought we know what the input is going to be --- all such places would  suddenly no longer run a primitive but a macro which in turn does some  additional processing)  c) there are some theoretical possibilities for a command to find out on its  own whether or not it is in a dangerous context (such as \edef) but i haven't  figured out a single solution so far that would run in acceptable time  (assuming that about every command would need to make this check over an over  again this is a significant overhead compared to the current approach where  the meaning of \protect is externally changed so that there is no or nearly  no penalty for testing the current state for the individual command)  d) is the current concept okay except for the problem that one needs to  educate people better about the use and misuse of \edef compared to  \protected@edef ? this is the current approach taken and somewhat the  opposite to b) if somebody would have ideas for a) or more specifically c) which is a subclass of a) i guess this would be great. if somebody would have a heart and would try to test b) on a large number of documents that would be great too and report about timing differences and failures found; essentially it means running the document with a format that contains additionally something like \let\@@edef\edef \def\protected@edef{ } \let\edef\protected@edef and ditto for \xdef and \protected@xdef and perhaps a few other commands in that part of the kernel if somebody would feel like writing a tugboat article on the current \protect i guess the TUB editor would be glad and so would i and probably a lot of people since this is a poorly understood concept (there is a bit of intro docu written long time ago in ltdefns.dtx but i unfortunately didn't got very far) have a go --- ( not just silently listen to this list ) frank ---- from pr/3084; see www.latex-project.org for the whole pr Subject: Re: latex/3084: New add@accent definition prohibits \edefs Date: Fri, 17 Sep 1999 11:31:38 +0200 Bernard,  > >The new definition of add@accent is supposed to keep correct  > >spacefactor's after accented uppercase letters. The cost is  > >that a command like \edef\agrave{\a} will not work as  > >expected (with respect to spacefactors).  > >More importantly, this makes the babel package fail,  > >and possibly others. you are right that the new definition of the internal accent handling made babel fail or rather made its use inside a straight \edef fail. problem is that LaTeX *never* supported arbitrary input inside straight \edef's. LaTeX goes a long way to make most commands robust (and if fragile provides \protect to protect them). However to make this work, LaTeX commands who accepts arbitrary input in their arguments are never allowed to pass them straight to an \edef. Instead they have to pass them to \protected@edef which defines \protect suitably to make everything work as expected. (note that in a straight \edef \protect does not help you as its default definition is \relax!) within standard LaTeX the new defnition of \ and friends still work perfectly in all circumstances. the fact that inside babel \edef was used in a place where \protected@edef should have been used was an oversight and is corrected. of course that doesn't help if other packages have commands that receive user input and pass it to \edef. but there is nothing that one can do about this other than fixing those packages. the fact that the accent commands are now failing inside such commands is a pity, but i don't think the conclusion can be to keep them broken in other respects just because there are potentially some broken packages that do not implement the protect mechanism of LaTeX correctly. it is simply a fact that to get the space factor right in case of \A one has to make an assignment and assignments are not expandable in TeX. so to make them work you have to avoid putting them into a primitive \edef. and even if they would not have that restriction and would work inside \edef there are plenty of other commands that have the same kind of problem inside an \edef. so the conclusion can only be:     DONT USE \edef unless you absolutely control its content. as i said, babel is now fixed again in this regard. hope this explains the general problem behind the anomaly you detected. best wishes frank`