AMG: Executing a script in a different stack frame means only that the script is given local access to that stack frame's variables, plus the attendant change to [info level]. It does not "make the caller do something." I know of only one circumstance where this distinction matters, but it's critical. uplevel return is the same as return with no uplevel; uplevel cannot be used to terminate the caller. (Instead use return's -level option.) PYK 2016-05-04: There's another effect to passing only one argument to

When to use [uplevel]

AMG: [uplevel] provides access to another stack frame's local variables. In most cases, [upvar] is preferred over [uplevel]: it's simpler and it doesn't interfere with bytecode compilation. However, if the script to be executed comes from elsewhere (e.g. it was passed as an argument), [uplevel] is required.

Harald Kirsch wrote in c.l.t: AMG: Executing a script in a different level means only that the script is given local access to that level's variables, plus the attendant change to info level. It does not "make the caller do something." I know of only one circumstance where this distinction matters, but it's critical: uplevel return is the same as return with no uplevel; uplevel cannot be used to terminate the caller. (Instead use return's -level option.) 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
 11

As 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
 44


[DGP] 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.
[AMG]: `uplevel 0 $script` is the same as `eval $script`.
[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. 
** Possible `uplevel` deficiencies **
'''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".... 
[AMG]: Here's a nice, simple [C]-style ''do ... while'' loop implemented using `uplevel`.  Other implementations are available on the [New Control Structures] page.
'''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} 
 test 

suchenwi:

 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 --> foo 

But 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 return 

D'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 break

kennykb: ... 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 GMT

kennykb: 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 -1

dgp: 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 -101

kennykb: 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 [L1 ] 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].

Possible [uplevel] deficiencies

AMG: Here's a nice, simple C-style do ... while loop implemented using [uplevel]. Other implementations are available on the New Control Structures page.

proc do {body while condition} {

    if {$while ne {while}} {
    if {$while ne "while"} {
    }
    uplevel 1 $body
    uplevel 1 [list while $condition $body]

}

I'm curious about the [bytecode] compilation properties of this [control structure].  I don't think `$condition` and `$body` ever get compiled.  Right?  If so, is there any way to fix this?  Putting the code to be executed into a randomly named [proc] would enable bytecode compilation, but running the code in a new level defeats the whole purpose of uplevel!
I'm curious about the [bytecode] compilation properties of this [control structure].  I don't think $condition and $body ever get compiled.  Right?  If so, is there any way to fix this?  Putting the code to be executed into a randomly named [proc] would enable bytecode compilation, but running the code in a new stack frame defeats the whole purpose of uplevel!
So, does there exist a [Tcl_Obj] type for bytecode-compiled script snippets which are '''not''' given their own level?
So, does there exist a [Tcl_Obj] type for bytecode-compiled script snippets which are '''not''' given their own stack frame?
I have another question.  Can `[do]` be enhanced such that `[return]` inside the `[do]` body (or condition!) will cause `[do]`'s caller to return?  Incrementing the `-level` option of `[return]` works, but this is not a change to `[do]`, it is a change to the arguments of `[do]`.
I have another question.  Can '''[[do]]''' be enhanced such that [[[return]]] inside the [[do]] body (or condition!) will cause [[do]]'s caller to return?  Incrementing return's -level option works, but this is not a change to [[do]], it is a change to [[do]]'s arguments.

proc test1 {} {

    do {return success} while {1}
    return failure

} proc test2 {} {

    do {return -level 2 success} while {1}
    return failure

} test1 ;# returns failure test2 ;# returns success

[AMG]: These are still burning questions...  Anyone have answers?

[steveb] Here is how I do this with Tcl8.5+ and Jim Tcl. Needs some small tweaks if you want to support break and continue properly.

proc do {body while condition} {
    if {$while ne {while}} {
    if {$while ne "while"} {
    }
    set rc [catch {
        uplevel 1 $body
        uplevel 1 [list while $condition $body]
    } msg opts]
    if {$rc} {
            dict incr opts -level
    }
    return {*}$opts $msg
}

DKF: With 8.6 (specifically, tailcall) you can do this:

See also

2002-10-17: returneval provides a Tcl-only implementation of something very similar to the above return -code eval.

• namespace
• upvar

RHS: TIP 90 doesn't cover the -code eval idea, does it? I read through it and it didn't seem to.