Version 26 of subst

Updated 2008-07-28 15:53:24 by dgp

http://www.purl.org/tcl/home/man/tcl8.5/TclCmd/subst.htm


What's the use of subst? As Joe English, who has deep experience in the area, explains, "[subst] is massively handy in text-processing applications, especially SGML and XML down-translators. [subst] and [ string map ] make Tcl particularly well-suited for this type of task." Many jobs for which Perl uses "right-hand-side" variables with regular expression substitution, Tcl does as REs whose results are subst-ituted.


Very simple example of using [subst] with XML/HTML.

       set html   {<html><head>$title</head></html>}
       set title  "Hello, World!"
       set output [subst -nocommands $html]
       set output
       <html><head>Hello, World!</head></html>

Another alternative would be to use XPath I use something like this in an application of mine. Of course the "text" of the 'html' variable above would probably come from a file.

I would much rather use XPath to hunt down the place to change.


RS most often uses subst for expanding Unicodes: cross-platform, in mostly 8-bit environments, it is most robust to output Unicodes in the \u.... notation - such snippets can be pasted into a text widget and visualized by

 subst [$t get 1.0 end]

[Explain the regsub idiom.]


03jun04 jcw - It would be useful to extend subst so it lets one catch variable accesses, and perhaps even command executions. What I mean is that when you subst text with "... $var ..." then sometimes it is useful to be able to intercept the expansion, by turning it into a call such as "myhandler var" for example, the result of which then gets used as substitution. The same (perhaps less important) might apply to "... [cmd ...] ..." expansions. This makes it simpler to implement tiny languages which also use $var and $var(item) as This makes it simpler to implement tiny languages which also use "$var" and "$var(item)" as

Would it be an idea to extend subst so it optionally passes each of its substitutions to a command? Could be a "-command ..." option, or simply the presence of more args.

DGP Am I missing something? Aren't you asking for variable and command traces? Which exist?

D'oh! I'm missing that in this case you want to set a trace on a whole set of variables/commands whose names you do not know. OK, something to think about...

Anyhow, I think that's the right way to address the issue generally... add more types of traces that can be used everywhere. I'd be shy about diverging the implementation of subst from the implementation diverging the implementation of subst from the implementation

jcw: Yes, that's exactly the scenario. subst on a string to expand names which are not known up front. Looks like there is no way to catch this right now. jcw - Yes, that's exactly the scenario. Subst on a string to expand names which are not known up front. Looks like there is no way to catch this right now. The key is to intercept between the parse for var/cmd expansions and the lookup for existing ones.


What changed in Tcl 8.4.0 with regards to how subst treats break and continue during command substitution?

See Tcl Bug 536831, Tcl Feature Request 684982, and the changes in the tests subst-10.*.

Without checking every byte, I think the incompatible changes are limited to those uses of subst that attempt command substitution on a string that is not a syntactically valid Tcl script -- arguably something no script should be doing anyway.