[I hope someone will provide some pointers to wiki and other resources relating to uplevel and when one would use it or should not use it.]
Harald Kirsch wrote in c.l.t: Question: Is it possible to have [macro] as a counterpart to proc such that [macro] defines a "function" which is evaluated in the caller's stack context, i.e., which does not create its own stack context?So how's this:
proc macro {name body} {
proc $name {} [list uplevel 1 $body]
set name ;# not needed, but maybe helpful
} ;# RS
0 % macro now {incr i; incr j}
now
0 % set j 10; set i 1
1
0 % now
11As a variation, you might also have the body executed in global scope, if you want to avoid global commands (not recommended normally, since the variables you use are not cleaned up): proc Sub {name body} {
proc $name {} [list uplevel #0 $body]
}
% Sub nenv {array size env}
% nenv
44DGP Note that [macro] will not be able to handle a body that includes a return command, due to some fundamental limitations in Tcl itself. See tcllib::control documentation for some more details.Tcl chatroom transcript:suchenwi: I'm not sure what you mean with "will not be able to handle a body that includes a return command." If there is one, it will cause the caller to return, I'd think. And that's like "#define BYE return;" in C - expected behavior for a macro.dgp: No, it won't. Try it.suchenwi: But Harald Kirsch already followed up, he wants macro to take arguments and have local variables. This is tricky indeed - you can't do a "downlevel"....dgp: Important to remember that uplevel adjusts only the command and variable substitution context. It can't really do anything to the calling stack.suchenwi: Ah - I see. Tried "macro foo {incr i; incr j;return;puts "oops"}".dgp: That doesn't test what needs testing.suchenwi: foo itself returns (the oops won't come), but does not terminate its caller. One would have to do
set body [string map {return {return -code return}} $body] ;-)dgp: macro mreturn return
proc test {} {puts a; mreturn; puts b}
testsuchenwi: proc macro {name body} {
proc $name {} [list uplevel 1 [string map {
return {return -code return}
} $body]]}dgp: Yes, that string map will handle more cases, but can't achieve [return -code error], etc. One way to fix this limitation would be to add a new status code to Tcl that caused the result to be evaluated in the caller's context by the caller. return -code eval {set var val}Then you would just map all [return $arg $arg ...] to return -code eval {return $arg $arg ...} . Not completely sure what cans of worms get opened by that new feature though. Of course, with that status code available, [macro] would be much easier to write too.kennykb: I like [return -code eval] -- except for deciding whether Tcl_EvalObj and its friends should interpret it or return it to the C caller.rmax: Hi Kevin. I think [return -code eval] would also help [do] to become complete for all possible cases.kennykb: Yes, dgp's comments make it obvious that it could do return -code eval { return -code eval { return -code whatever {... }}} to break out of multiple scopes.dgp: I still would not recommend that though. Just as it should always be [uplevel 1 ...].kennykb: Don: I wouldn't recommend it either, but a certain depth of nesting is needed for [control::do] to handle [return -code] within the loop body, right?rmax: You won't of course create nested [return -code eval] constructs yourself, but they could emerge from nested [do] loops or macro calls.dgp: I think catch needs extending too. catch {return -code error foo} msg --> 2
set msg --> fooBut where is the '1'? Add another argument to catch to get the '1'.kennykb: Yes, also need a place to stash the -errorcode and -errorInfo options to [return].dgp: I think you can just grab them from ::errorInfo and ::errorCode directly.dgp: ... but yes, status code '1' needs special handling, as always, to deal with ::errorInfo and ::errorCode. For the rest, set code {catch $body msg} if {$code == 2} {
# We caught a returnD'oh! Make that set code {catch $body msg returncode}then...return -code eval [list return -code $returncode $msg] }dgp: Ah! I think I see what you were getting at!kennykb: Hmmmm, it occurs to me that [return -code eval] subsumes all the others.dgp: If $body itself contains [return -code eval $script].kennykb: return -code error -errorcode code -errorinfo into message => return -code eval [list error message info code]return -code break => return -code eval breakkennykb: ... and the same for continue and return.suchenwi: Looks like it would actually make the language smaller....dgp: ...except maybe for the differences in ::errorInfo construction.kennykb: We'd have to keep the other syntax on return for back compatibility, but I think it could simplify the mechanism. Oh, yeah, ::errorInfo construction. OK, make it return -code eval [list error $message [doTheRightThingWith $info] $code].suchenwi: ..and if eval is the last remaining value for -code, one could shorten that to -eval...kennykb: Hmm, need a new command for those crazy enough to do [return -code 6].dgp: How about "-code ok" ? I guess that's the default case where no eval script is done. Note: that's different from eval of an empty script.Why a new command?kennykb: I suppose if you wanted to be obsessive about it, you could say return -code ok $value -> return -code eval [list identity $value] where identity is a command that returns its arg, but I wouldn't go that far, myself.dgp: We still need [return -code 6] if we want ability to create new status codes at the script level.rmax: Isn't this possible to handle completely on the catch side? Doing it on the return side involves string operations on scripts, which hurts performance.dgp: proc niftyNewBreak {} {return -code 6}kennykb: proc niftyNewBreak {} { return -code eval {::tcl::raiseException 6 }} ? 8^)rmax: ... and there is the danger of substituting a "return" that isn't really a return.dgp: Ah, I see.kennykb: Where are we doing string operations on scripts?dgp: No substitution. catch and re-raise.rmax: The implementation of [do] would have to substitute "return -code ..." by "return -code eval {return -code ...}" or am I wrong?kennykb: Reinhard: Ah, but there's no string processing there. The new command is a pure list.dgp: [do] includes the line:
set code [[catch { uplevel 1 $body } result]]Change to: set code [catch { uplevel 1 $body } result returnCode]Add a case to the switch 2 {return -code eval [list return -code $returnCode $result]}kennykb: Yes.. and note that the thing to be 'eval'ed is a pure list.dgp: The use of eval might prevent bytecode, so that's a performance issue, but there should be no need to reparse the strings.rmax: wouldn't "2 {return -code $returnCode $result}" be sufficient?dgp: No. The idea is that the return in $body should act as if it was evaluated in the caller. Hmmm. so far have taken care of the first pass through [do]. Remaining passes are handled by an [uplevel 1 {while ...}] That probably needs similar handling. Yes. Same changes there.rmax: I still think it would be sufficient for [do] to be able to catch and pass on the -code argument to return.kennykb: Gentleb[e]ings, I think we're excising one of the language's major warts here.rmax: Can you give an example of usage for [do] where that wouldn't be enough?* clock : 1011891602 : Jan 24,2002 Thursday 5pm GMTkennykb: What if the body of the [do] contains [return -code eval] ?dgp: I think that case is handled above.rmax: do will pass on [return -code eval].dgp: rmax: you don't want to return from [do], you want to return from the caller of [do].rmax: Yes: do {
return -code return
}
inside do:
set code [catch { uplevel 1 $body } result retCode]
...
2 { return -code [list return $retCode] $result }What about that?dgp: That will return from the caller of [do] when what is intended is to return from the caller of the caller of [do].* rmax is totally confused now... has to try some things...kennykb: That's yet another new syntax for return. How is that preferable to "return -code eval [list return -code $retCode $result]" ?dgp: Oh. I mis-read part of that last one. Given what we've described, that should be an error:rmax: it is a bit shorter, and I haven't thought much about it...dgp: unknown code: "return 2"kennykb: Since we need [return -code eval] anyway -- in order to implement [macro], I'm reluctant to invent still more syntax just to make [do] a tiny bit shorter.rmax: Yep, Don. It implied another proposal of changing the syntax of return. OK, Kevin, you win.dgp: The numeric code for eval should be -1.kennykb: Why -1?dgp: #define TCL_EVAL -1dgp: It's an interesting status code in that it is completely internal. No command will actually return it, it will return the result of eval'ing the result instead. [return -code $integer] has been documented to allow any postive integer. -1 won't interfere with previous extended status codes.kennykb: OK, we have a compatibility issue with C code that calls commandProc's directly. (There are examples doing stuff like that in Brent's book, IIRC.)dgp: hmmmm... take that back. I guess "positive" isn't in the docs after all. Are there any example doing that with *ObjCmd's?kennykb: I don't think we have too much of a problem as long as we avoid whatever code exp_continue uses.dgp: Any existing code is calling existing command procedures. None of which return the status code TCL_EVAL, so is there really a problem?bbh: exp_continue seems to be -101kennykb: Don: Doesn't Brent's code have a Tcl_GetCommandInfo so that it's actually calling a user-supplied command proc?dgp: Anyhow look for a section "Bypassing Tcl_Eval". His code doesn't handle any extended return codes, so there's no new problem here. His routine is called Tcl_Invoke. Tcl_Invoke("break", NULL); would cause problems now.* suchenwi has to stop mirroring this debate on http://wiki.tcl.tk/1507 - real work calling...TIP #90 [1] is a formalized follow-up of the above discussion. ...and is now incorporated in Tcl 8.5 development.17 October 2002: returneval provides a Tcl-only implementation of something very similar to the above [return -code eval].RHS TIP 90 doesn't cover the -code eval idea, does it? I read through it and it didn't seem to.Lars H: No, but -code eval was considered as an alternative solution. Maybe it's not in the TIP itself, but in some of the surrounding discussion, however a good deal of thought was put into it.
AMG: [uplevel 0 $script] is the same as [eval $script].
See also: