'''`[http://www.tcl.tk/man/tcl/TclCmd/subst.htm%|%subst]`''', a [Tcl Commands%|%built-in] Tcl [command], [dodekalogue%|%subtitutes] variables Commands%|%build-int] Tcl [command], [dodekalogue%|%subtitutes] variables ** Synopsis ** : '''subst''' ?'''-nobackslashes'''? ?'''-nocommands'''? ?'''-novariables'''? ''string'' ** Documentation ** [http://www.tcl.tk/man/tcl/TclCmd/subst.htm%|%official manpage%|%]: ** Description ** '''`subst`''' performs the first stage of Tcl script evaluation, i.e. '''`subst`''' performs the first stage of Tcl script evalution, performing evaluating the commands contained at the top level of the script. evaluating the commands contained at the top level of the script. According to [Joe English]: `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 [regular expression%|%REs] whose results are subst-ituted. ** Examples ** Simple example of using `subst` with XML/[HTML]. ====== set html {$title} set title "Hello, World!" set output [subst -nocommands $html] set output ;# -> Hello, World! ====== Another alternative would be to use [XPath] ** Discussion ** [RS] most often uses `subst` for expanding [Unicode%|%Unicodes]: cross-platform, in mostly 8-bit environments, it is most robust to output Unicodes in the `\u....` notation - such snippets can be pasted into a text widget and visualized by ====== subst [$t get 1.0 end] ====== ** Variable Substitution ** Even when the `-nocommands` option is used, variable substitution triggers any command substitutions necessary to complete the variable substitution: ======none set var "code inclusion perverse \$tcl_platform(os\[puts OUCH!\])" puts [subst -nocommands $var] ==> OUCH! ====== reference: [http://groups.google.fr/group/fr.comp.lang.tcl/msg/b0038cac3c0cfa04%|%Eric Hassold, fr.comp.lang.tcl, 2008-12-30%|%] reference: [http://groups.google.fr/group/fr.comp.lang.tcl/msg/b0038cac3c0cfa04%|%Eric Hassold ,fr.comp.lang.tcl ,2008-12-30%|%] reference: [http://sourceforge.net/tracker/?func=detail&aid=536838&group_id=10894&atid=110894%|%Tcl bug 536838%|%] -- Ok, I saw [http://core.tcl.tk/tcl/tktview?name=536831%|%Bug 536831] above. I think a '''big warning''' should be inserted in the manual. [Lars H]: What has Bug 536831 to do with this? I see nothing about -nocommands in that report. The problem with `[puts] OUCH!` rather seems to be that variable substitution can trigger command substitution in the array index part, or to put it differently, once one type of substitution has triggered, [subst] has no control of what happens until that substitution is complete: ======none % subst -nobackslashes {$tcl_platform(threade\x64)\x64} 1\x64 ====== A warning indeed seems appropriate. ** History ** What changed in Tcl 8.4.0 with regards to how subst treats break and continue during command substitution? See Tcl Bug 536831, [http://core.tcl.tk/tcl/tktview?name=684982%|%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. ** Enhancement Suggestion: Hook for Variable Expansion ** [jcw] 2004-05-03: 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 access mechanisms, but to things which are not necessarily stored in Tcl variables/arrays. 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 [trace]s? 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 of the substitution portion of normal script evaluation. [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. Perhaps some new "unknown traces" (or whatever terminology) would indeed be better. The key is to intercept between the parse for var/cmd expansions and the lookup for existing ones. ** Parallel to double quoting ** [AMG]: Am I correct in my understanding that: ====== [subst {anything at all}] ====== is always equivalent to: ====== "anything at all" ====== for absolutely any value of "anything at all"? It occurred to me that if this is indeed the case, then maybe this equivalence could be the reason why backslash-newline-whitespace inside braces is replaced with a single space, in the interest of mirroring the way double quotes work. But then I experimented and found that `subst` internally will do this replacement and does not need the Tcl interpreter to preprocess its input in this way. [PYK]: Could you elaborate on what you mean by "internally will do this replacement"? [AMG]: `subst` internally replaces backslash-newline-whitespace with a single space. It's as simple as that. Here's a demonstration: ====== % subst abc\\\n\ \ def abc def ====== By the way, I really did mean to put brackets around my first example. I'll let you take a moment to think about why. [PYK]: I see it now. The two command names only resolve to the same command if they are identical values. I was looking at the examples not as scripts but as snippets from some individual command. ** See also ** [AMG]: I wouldn't use the term "command" to describe what's happening here, since that term already has specific meaning in Tcl distinct from what's being discussed. Yes, these are intended to not be complete scripts or even complete command invocations, but rather snippets from some individual command, though let's use a term more precise than snippet: [word]. [PYK]: Oops, my bad. I guess I meant ''[dodekalogue%|%command procedure]''! But in a way, removing the brackets actually makes more sense, because then the two examples above can be interpreted as commands, and the meaning of "equivalent" becomes clear: "identical value", via Tcl's own command resolution operation. [AMG]: No really, these are not commands, they are word constructors. [eval]: [regsub]: [string map]: [an extension to subst]: [Template and Macro processing]: <> Arts and crafts of Tcl-Tk programming | Tcl syntax | Command | String Processing