'''[http://www.tcl.tk/man/tcl/TclCmd/tailcall.htm%|%tailcall]''', a [Tcl Commands%|%built-in] [Tcl] command, executes a command in place of the current command. ** Synopsis ** : '''tailcall''' ''command'' ?''arg''...? ** See Also ** [Tail call optimization]: [TIP]#[http://tip.tcl.tk/327%|%327]: [wrapping commands]: wrap commands by using `[interp invokehidden]` together with `tailcall` ** Description ** '''`tailcall`''' interprets its arguments as a command and executes the command,replacing the [stack frame%|%execution frame] of the command that invoked `tailcall`. Unlike `[uplevel]`, it does not [eval%|%evaluate] its arguments as a script, so [double substitution] does not occur. Unlike some other languages, `tailcall` is not limited to executing only its caller, but can execute any command. The command to be executed is resolved in the current context before `tailcall` replaces the context. `tailcall` is made possible by [NRE]. It first became available as `[::tcl::unsupported::tailcall]` in the release of Tcl8.6a2. Contrast the following two commands: ====== tailcall foo [bar] $var return [uplevel 1 [list foo [bar] $var1]] ====== There are a couple of differences: 1. '''foo''' is resolved in the ''current'' context, not in the caller's 1. the stack frame is ''really'' gone, not just ''virtually''. This has positive effects on memory, and a possibly confusing effect on stack traces. To `tailcall` a script: ====== tailcall try $script ====== ---- [WHD]: Let me see if I understand what this does. ====== proc fred {} { george } proc george {} { tailcall harry } ====== If I call fred, it's almost as though fred called harry directly, instead of george. Not so? [MS]: yup - all traces of george are gone from the program stack when harry is called. Now, if harry resolves to a different command in george's current namespace than it would under fred's, the harry that is called is george's and not fred's (no diff if the commands are FQ, of course). I think this does pretty much what delegation is supposed to do, right? ---- [jima] 2009-10-15: Perhaps this has been asked before or somewhere else... Is this an optimization that takes place at bytecode generation time? I mean, once fred knows that has to call harry directly the bytecodes generated would be the ones equivalent to have said: ====== proc fred {} { harry } ====== I reckon I am not familiar with all the internals of Tcl but I find this would be an interesting thing. Wouldn't this be a new way to have some sort of macros? [MS]: Currently, `tailcall` is not bytecompiled. Everything happens at runtime. That extremely simple example could indeed be bytecoded in a minute, but things get more involved as soon as `fred` has a bit more structure to it: arguments, local variables, namespace issues both for variable and command lookup, multiple exit points with different (or no) `tailcall` in them, etc. [jima]: Thanks a lot Miguel for the answer. I see the point. I guess this is the same with `[uplevel] 1`, isn't it? ====== proc fred {} { uplevel 1 { #code here } } ====== Would it be interesting to define a case (like a contract) saying if your proc is simple enough then it gets bytecompiled and you get some benefits? [MS]: you do not mean "bytecompiled" but rather "inlined into the caller", as all `[proc]` bodies get bytecompiled. There are quite a few other issues with that, especially to accomodate Tcl's dynamic nature. Changing one inlined proc would cause a spoiling of all bytecodes and recompilation of the world, at least with the current approach to bytecode lifetime management. ---- [AMG]: Sounds a lot like `exec` in [Unix shells]. See [execline] for more information on a noninteractive Unix shell where everything is done with exec/tailcall. ---- [PYK] 2015-12-06: Combine `tailcall` with an [identity function%|%identity] command to emulate `[return]`: ====== proc p1 {} { tailcall lindex {Hello from p1} } ====== ** Interaction with `[try]` ** ===none '''%''' proc foo {} {puts {I'm foo}} '''%''' proc bar {} {puts {I'm bar}; try {tailcall foo} finally {puts exiting}} '''%''' foo ''I'm foo'' '''%''' bar ''I'm bar'' ''exiting'' ''I'm foo'' === 31-03-2015 [HE] I'm sure ;-) that I don't understood what happend there. Why "exiting" is printed before "I'm foo" when I call bar? If I change bar to ====== proc bar {} {puts {I'm bar}; try {puts tryBody; tailcall foo} finally {puts exiting}; puts afterwards} ====== and call it, I get: ======none I'm bar tryBody exiting I'm foo ====== What I see is that tailcall replace the rest of proc even inside the body of try. But then, why is the finally clause executed? And even, if we assume the finally clause has to be executed because it is documented always to be executed, then there would be the question, why before the execution of the tailcall command? [AMG]: [[foo]] is invoked by replacing [[bar]] which implies the intervening [[[try]]] block must exit before [[foo]] can start. ---- [wdb]: Apparently, the `tailcall` closes one of the last gaps in Tcl: Tail recursion as known in [Scheme]. ** Example: Cause Caller to Return ** ====== proc one {} { two return 8 } proc two {} { tailcall return 5 } one ;# -> 5 ====== `one` returns `5`, not `8`, because by invoking two, which, through `[tailcall]`, is replaced by `[return]`. ** Example: Factorial ** [NEM]: As a test/demo of how to use this facility, here is a simple benchmark using the factorial function: ====== package require Tcl 8.6a1 namespace import ::tcl::mathop::* interp alias {} tailcall {} tcl::unsupported::tailcall # Naive recursive factorial function proc fac n { if {$n <= 1} { return 1 } else { * $n [fac [- $n 1]] } } # Tail-recursive factorial proc fac-tr {n {k 1}} { if {$n <= 1} { return $k } else { tailcall fac-tr [- $n 1] [* $n $k] } } # Iterative factorial proc fac-i n { for {set k 1} {$n > 1} {incr n -1} { set k [expr {$n*$k}] } return $k } proc test {} { set fmt {%-10s ..%-12.12s %s} puts [format $fmt Implementation Result Time] foreach n {1 5 10 100 500 1000 2500 5000 10000} { puts "\nfac $n:" foreach impl {fac fac-i fac-tr} { if {[catch {$impl $n} result]} { set result n/a set time n/a } else { set time [time [list $impl $n] 10] } puts [format $fmt $impl $result $time] } } } test ====== Putting this in a table, we get (timings taken on Linux box, 2.66GHz, 1GB RAM): %| N | `fac` Time | `fac-i` Time | `fac-tr` Time |% &| 1 | 3.2 | 3.0 | 2.8 |& &| 5 | 10.1 | 4.7 | 19.4 |& &| 10 | 18.4 | 6.4 | 37.9 |& &| 100 | 345.5 | 267.4 | 717.8 |& &| 500 | 3133.9 | 3715.6 | 6182.5 |& &| 1000 | n/a | 13811.7 | 19764.3 |& &| 2500 | n/a | 65121.1 | 84556.5 |& &| 5000 | n/a | 241176.8 | 288136.1 |& &| 10000 | n/a | 987057.8 | 1643480.7 |& As we can see, the tail-recursive version is slightly slower than the iterative version, and unlike the naive version, manages to not blow the stack. ** Using Tailcall for Callbacks ** [Lars H] 2010-05-09: As of late, when writing an `[uplevel]`, I've sometimes found myself thinking "That would be slicker with `[tailcall]`, but I can't rely on 8.6 features in this project". Today it occurred to me that one can however use a `[proc]` to emulate the properties of `tailcall` that would be needed in these cases, and thus provide a route for forward compatibility. The main situation I've encountered is that of delegating to another command which may make use of `[upvar]` or `[uplevel]`. That's basically taken care of by ====== proc utailcall args {uplevel 2 $args} ====== although it's safer to make it ====== proc utailcall args {return -code return [uplevel 2 $args]} ====== in case the "terminate proc early" aspect of `tailcall` is relied upon; this is easy to do without thinking much about it. Another aspect of `tailcall` is the name resolution of the called command. This can be done as follows ====== proc ntailcall {cmd args} { return -code return [ [uplevel 1 [list ::namespace which $cmd]] {*}$args ] } ====== but it's almost as easy to do both at the same time ====== proc untailcall {cmd args} { return -code return [ uplevel 2 [list [uplevel 1 [list ::namespace which $cmd]]] $args ] } ====== A word of warning here is that this will produce a very confusing error message if the command is undefined, as `[namespace which]` returns an empty string in that case. A third aspect is that of preserving `[return]` levels. ====== proc rtailcall args { catch $args result options dict incr options -level 2 return -options $options $result } ====== This leaves some extra material in the [errorInfo], but one can probably live with that. Combining the "r" and "u" aspects is straightforward, but will leave even more: ====== proc rutailcall args { catch {uplevel 2 $args} result options dict incr options -level 2 return -options $options $result } ====== To complete the set, one might just as well write down the combination of the "r" and "n" aspects ====== proc rntailcall {cmd args} { catch { [uplevel 1 [list ::namespace which $cmd]] {*}$args } result options dict incr options -level 2 return -options $options $result } ====== and of all three ====== proc rnutailcall {cmd args} { catch { uplevel 2 [list [uplevel 1 [list ::namespace which $cmd]]] $args } result options dict incr options -level 2 return -options $options $result } ====== But note: ''all of the above will fail if used for tail recursion'', as soon as the loops get long enough. ** Replacement for `[uplevel]` ** [AMG]: `[uplevel]` has limitations with respect to [bytecode] compilation and interpretation of `[return]`. If `[uplevel]`'s level count is `1`, and if it's the last thing being done in the `[proc]`, these limitations can be avoided by using `[tailcall]` instead. Note that `[uplevel]` takes a script whereas `[tailcall]` takes a command. If you want to pass a script to `[tailcall]`, make it be the sole argument to `[try]`. See [http://wiki.tcl.tk/1507#pagetocc0434a60%|%Possible uplevel deficiencies%|%]. Also see [http://wiki.tcl.tk/1507#pagetocb7539876%|%When to use uplevel] for more on when to use or avoid `[uplevel]`. See [http://wiki.tcl.tk/1017#pagetoc74fae1d9%|%eval vs bytecode] for discussion and performance numbers regarding [bytecode] compilation with `[eval]`, `[uplevel]`, `[try]`, and others. ** When to apply `tailcall` optimization ** [HaO] 2012-12-14: Is it a good idea to replace any code: ====== proc proc1 {arg1 arg2} { # do something here which finds arg3 and arg4 return [proc2 $arg3 $arg4] } ====== by ====== proc proc1 {arg1 arg2} { # do something here which finds arg3 and arg4 tailcall proc2 $arg3 $arg4 } ====== If proc2 is for sure found in the caller namespace? Is this an intelligent optimization? I came to this idea, as the TI C compiler calls this "tailcall optimization". [AMG]: Yes, except in a highly unlikely situation where `proc2` needs `proc1` to be visible in the stack. Procedures really ought not to care who called them, but Tcl makes all sorts of things possible, including stupid things. ** Misc ** [NEM]: Many thanks to [MS] for his hard work making this a reality! <> Command | Concept | Control structure | Functional Programming