'''[http://www.tcl.tk/man/tcl/TclCmd/return.htm%|%return]''', a [Tcl Commands%|%built-in] [Tcl] [command], returns a value, a code and other options from a particular [level]. ** Synopsis ** : `'''return''' ?''result''` : `'''return''' ?'''-code''' ''code''? ?''result''` : `'''return''' ?''option value''? ?''result''` ** Options ** `'''-errorcode''' ''list''`: `'''-errorinfo''' ''info''`: `'''-errorstack''' ''list''`: added in Tcl8.6 `'''-level''' ''level''`: `'''-options''' ''options''`: ** Documentation ** [http://www.tcl.tk/man/tcl/TclCmd/return.htm%|%official reference]: [TIP] [http://www.tcl.tk/cgi-bin/tct/tip/90.html%|%90], Enable `return -code` in Control Structure Procs: ** Description ** Delivers a return code for some level to the interpreter, which then takes some action based on the received code. By far the most common use case is to return a value from the current procedure: ====== proc knightswhosay {} { return Ni! } ====== '''`-level`''' is a positive integer indicating the number of levels up from the current level. Level `0` is the evaluation of `return` itself, level `1` is the caller of return, and so on. '''`-code`''' is a positive integer or the symbolic value as specified later on this page. By default, the value of `-code` is `1` (`ok`) and the value of `-level` is `1`. The following three commands all effectively instruct the caller to '''return normally from the current procedure (level 1)''': ====== return return -level 0 -code return return -level 1 -code ok ====== The return value is either the final argument or the [empty string]. `[catch]` and `[try]` can be used to intercept a return. Any additional ''option''/''value'' pairs are added to the options dictionary for the level. '''[[TODO: Document Tcl 8.5's extended handling]]''' ** Return Codes ** Typically, `'''-code'''` isn't specified and the level returns a normally, i.e. with a completion code of `TCL_OK`. However, `-code` may be any of the following numeric or alphabetic values, '''`0`''': '''`ok`''': Success. `return -level 1 -code ok` is the same as `[return]`. (TCL_OK) '''`1`''': '''`error`''': An error. `return -level 0 -code error` is the same as `[error]`. (TCL_ERROR) '''`2`''': '''`return`''': Cause the level up to return. `return -level 0 -code return` has the same effect as `[return]`, as well as `return -level 1 -code ok`. (TCL_RETURN). '''`3`''': '''`break`''': Instruct the next level up break out of the innermost nested loop in the current script. `return -level 0 -code break` is the same as `[break]`. (TCL_BREAK) '''`4`''': '''`continue`''': Instruct the next level up to terminate the current iteration of the innermost nested loop in the code that invoked the current procedure and start the next iteration. (TCL_CONTINUE) ''`value`'': Any integer. User-defined behaviour. Rarely used with values outside the range 0..4. `-code` is rarely used, as commands such as `[error]`, `[break]` and `[continue]` handle the common cases. The less common case is that a procedure implements a [new control structures%|%new control structure] and needs act, within its caller, like `[break]`, `[continue]`, or `[error]`. In other words, it needs `return -level 1 ...` instead of `return -level 0 ...`. Another use case for `-code` is to have the caller, rather than the current procedure, report that the wrong number of arguments were provided, since this makes the error message more clear. Again, what's needed is `return -level 1 -code error` instead of `return -level 0 -code error`. Two additional options, '''`-errorinfo`''' and '''`-errorcode`''', may be used to provide additional information during error returns. These options are ignored unless `-code` is `error`. '''`-errorinfo`''' specifies an initial stack trace for `[errorInfo%|%$errorInfo]`. If it is not specified, the stack trace left in `[errorInfo%|%$errorInfo]` will include the call to the procedure and higher levels on the stack but it will not include any information about the context of the error within the procedure. Typically the `[info]` value is supplied from the value left in `[errorInfo%|%$errorInfo]` when `[catch]` traps an error within the procedure. If '''`-errorcode`''' is specified, ''`list`'' provides a value for `[errorCode%|%$errorCode]`. Otherwise, `[errorCode%|%$errorCode]` will default to `NONE`. (from: [Tcl Help]) ** Hack: Arbitrary Data after `return` ** After `return`, your script can contain whatever, for instance comments: ====== proc foo {} { puts Foo return This is not Tcl - code after the return is never evaluated so may be used for commenting... } ;# RS ====== [DGP]: In Tcl 7 and in recent enough Tcl 8.5 that is correct. In the releases in between, due to some limitations in the [bytecode] compiler/execution machinery it could not be "whatever": * braces still needed to be balanced * Commands like `[set]` get byte-compiled early, so a syntax error is found if a line in that post-return comment starts with set and has more than two other words. [Joe English] also disagrees that "whatever" can appear after `return`. `[proc]` interprets its third argument as a script. It's therefore unwise, and one could argue even illegal, to pass in something that's not at least syntactically valid as a script, even if you know that parts of it will never be executed. By way of analogy: `[lindex]` interprets its first argument as a list, so you'd better only pass it valid lists. In Tcl 7.6 and earlier you could actually get away with things like ====== lindex "a b c {bad{list" 1 ====== as long as the ''examined'' part of the list was syntactically valid. However, this was more of an accidental artifact of implementation details than anything guaranteed by the language, and in fact this raises an error in more recent Tcl versions. Similarly, if a command expects a script, you'd better pass it a script. [PYK] 2013-12-10: However, if `[lindex]` is missing its second value, the first value can still be any value, even one that isn't a valid [list]. [AMG]: There's a reason for this. When [[lindex]] is given only one argument, it interprets that as instruction to not perform any list indexing. No list indexing means no requirement to be a list. It's that simple. [jenglish]'s statement is correct, though it's more philosophical than practical. If [[return]] is redefined, code following the [[return]] could certainly come into play. Or perhaps the code isn't being executed but rather is being analyzed by [Nagelfar], which will surely take issue with the invalidity of the code. One real possibility which would upset Tcl in any event is if the text following [[return]] contains mismatched braces. And last, I really ought to mention that the first time Tcl runs the proc, it tries to [bytecode] the whole thing, which means wasting time analyzing garbage. If there's any doubt about the finality of the [[return]] (e.g. a conditional is in play), the compiled proc will contain code to return any errors found in parsing. ====== proc moo {x} { if {$x} {return 5} "invalid"asdf } % tcl::unsupported::disassemble proc moo ByteCode 0x0x913eb0, refCt 1, epoch 15, interp 0x0x8c6ac0 (epoch 15) Source "\nif {$x} {return 5}\n\"invalid\"a"... Cmds 2, src 34, inst 26, litObjs 4, aux 0, stkDepth 2, code/src 0.00 Proc 0x0x95b5e0, refCt 1, args 1, compiled locals 1 slot 0, scalar, arg, "x" Commands 2: 1: pc 0-11, src 1-18 2: pc 4-6, src 10-17 Command 1: "if {$x} {return 5}"... (0) loadScalar1 %v0 # var "x" (2) jumpFalse1 +7 # pc 9 Command 2: "return 5"... (4) push1 0 # "5" (6) done (7) nop (8) nop (9) push1 1 # "" (11) pop (12) push1 2 # "extra characters after close-quote" (14) push1 3 # "-code 1 -level 0 -errorcode NONE -errori"... (16) syntax +1 0 (25) done ====== ** `return` from `[source]` ** The fact that `return` also terminates `[source]` can be used for loading `[array]` contents without specifying an array name. Let the file t.tcl contain: ====== return { one 1 two 2 three 3 } ====== Then you can write it like this: ====== array set myArrayName [source t.tcl] ;# RS ====== [wdb]: This works, but being a purist, I prefer this text in the file to `[source]`: ====== list one 1 two 2 three 3 ====== [RS] 2006-06-23: sure. Just if you have hundreds and thousands of array elements, with `[list]` you'd have to backslash-escape the newlines, while with bracing they need not. [DKF]: I'm of the kind of purist which doesn't insist on using [list] to indicate listishness; that's an illusion and the [list]-compiler knows it. ** Use in [pkgIndex.tcl] ** See [package index script interface guidelines] for another use of `return` in `[source]`'d scripts: The main use for `return` outside procedures is in [pkgIndex.tcl]: ====== if {![package vsatisfies [package provide Tcl] 8.4]} return ====== which avoids presenting the package to [interp]s that cannot use it. ** `return` -code error ** [RS] 2005-08-08: Using `return -code error` in place of plain `[error]`, you get a leaner error traceback which is possibly better to read: ======none % proc 1 x {if {$x<=0} {error "too small"}} % proc 2 x {if {$x<=0} {return -code error "too small"}} % 1 0 too small % set errorInfo too small while executing "error "too small"" (procedure "1" line 1) invoked from within "1 0" % 2 0 too small % set errorInfo too small while executing "2 0" ====== [DKF]: I find this is useful when distinguishing between problems with the values passed in by the caller (such as if the code really wants a string of length 37, has documented this, and yet the user has given a string of length 28), and problems internal to the code that it isn't reasonable for the caller to try to get right. ** `return` to the Current Level ** [AMG] [PYK]: `return -level 0 $x` simply sets the interpreter result to `$x`. This is the canonical [identity function]; see the linked page for more information. ** Cancel an Error ** [HaO]: When a return code should be forwarded to the caller, one could remove the `level 0` to not directly trigger an eventual exception here: ====== if {[catch {script goes here} err options]} { dict unset options -level return -options $options } ====== This was useful to me in the context of [Tk] `[bind]` scripts, which return `break` if no further bind scripts should process. ** `return` vs falling off the end of the proc ** [PYK] 2016-09-15: Historically, `[return]` at the end of a procedure was slightly more performant than a final command that didn't explicitly `return`. This isn't the case in modern versions of Tcl, which employ [bytecode%|%byte compilation]. ** Related Commands ** `[source]`: `[eval]`: `[proc]`: `[uplevel]`: includes a TclChat discussion on the future of `return` `[break]`: `[continue]`: `[error]`: `[throw]`: `[yield]`: `[yieldm]`: `[yieldTo]`: `[tailcall]` is also related to `return` in that it terminates execution of the current `[proc]`. However, unlike `return`, `[tailcall]`'s [continuation] is not the caller. `[yieldTo]` is also related to `[tailcall]` in that it has a custom continuation. ** See Also ** [Errors management]: [namespace eval]: [Funky Tcl extensibility]: tricks to play with `return -code return` and `[error]` on `return -code error` [try ... finally ...]: [KBK] 2001-01-02: how to use `return -code` to implement a new control structure. [Lars H]: Other pages which do that kind of thing are `[breakeval]` (using `-code 10`) and `[returneval]` (using `-code -1`). 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