'''[http://www.tcl.tk/man/tcl/TclCmd/eval.htm%|%eval]''', a [Tcl Commands%|%built-in] Tcl [command], interprets its arguments as a [script], which it then evaluates. ** Synopsis ** : '''eval''' ''arg'' ?''arg ...''? [Injection Attack]: [Introspection]: [Many Ways to eval]: [namespace eval]: [Tcl Quoting]: [try]: can be used as [bytecode%|%byte-coded] alternative to `eval` [http://www.tcl.tk/man/tcl/TclCmd/eval.htm%|%official reference]: ** Description ** '''`eval`''' concatenates its arguments in the same fashion as `[concat]`, and hands them to the interpreter to be evaluated as a Tcl script in the scope of the caller of `eval`. It then returns the result of that evaluation or any error generated by it. `eval` is an old, old command that's been in [Tcl] from the beginning. `eval` is useful when one wishes to generate a script and then interpret it. In modern Tcl, `eval` is merely an abbreviation for `[uplevel] 0`. This is reminiscent of code-data duality, such languages as [Lisp] and [Forth], '''self-modifying programs''', and related esoterica. `[if] 1 ...`, which may be [bytecode%|%byte-compiled], is an efficient replacement for `eval` in all cases. `eval` itself may also be byte-compiled if it is given only one argument which is a literal string. To avoid the caveats of `eval` the rule of thumb is to pass only one argument to `eval`. Passing a single script rather than relying on implicit concatenation helps to avoid dumb mistakes such as passing a string which contains newlines. Also, when there is only one argument, and that argument is a [pure list], `eval` can avoid generating any string representation for the value ([https://groups.google.com/d/msg/comp.lang.tcl/DaPwbIcBFEI/VpTk_sjk0l8J%|%tcl objc type: args uses only string of own type], [comp.lang.tcl], 2002-06-19). which contains newlines. Also, when there is only 1 arguement, and that argument is a [pure list], `eval` can avoid generating any string representation for the value ([https://groups.google.com/d/msg/comp.lang.tcl/DaPwbIcBFEI/VpTk_sjk0l8J%|%tcl objc type: args uses only string of own type], [comp.lang.tcl], 2002-06-19). Because the script is evaluated in the scope of the caller of `eval` a terminating command like `[return]` will cause the caller of `eval` to return. `eval` is used by all the commands -- `[bind]`, everything with `-command`, and so on -- that receive scripts as arguments. When using `eval`, it is very easy to leave holes which can be exploited to [injection attack%|%inject] malicious code, so it is best to exercise extreme care with it until one learns the ropes. `eval` can often be avoided, particularly with more modern recent versions of Tcl. ** Eval and [double substitution] ** `eval` is one of the Tcl constructs that causes the Tcl interpreter to scan over its arguments another time and evaluate what it scans. Given the following assignment, ====== set b {the total is $20} ====== each of the following commands returns a is different result: ====== set a $b eval {set a $b} eval "set a $b" eval [list set a $b] ====== Consider the first one, ====== set a $b ====== Prior to invoking `[set]`, Tcl performs its substitutions, so the command becomes ====== set a {the total is $20} ====== `[set]` receives the arguments `a`, and `the total is $20`, does not perform any internal parsing or substitution on its own, and sets the value of `$a` to `the total is $20`. In the second one, ====== eval {set a $b} ====== Tcl does not perform any substitutions on the value in curly brackets, so the command remains unchanged. `eval` receives one argument, ====== set a $b ====== which it then hands back to the interpreter. This time the interpreter sees `set a $b`, which becomes: ====== set a {the total is $20} ====== so the value of `$a` becomes `the total is $20`. In the third example, ====== eval "set a $b" ====== Tcl changes the command to ====== eval {set a the total is $20} ====== `eval` receives the argument ====== set a the total is $20 ====== and hands it to the interpreter for evaluation. This time, the interpreter sees ====== set a the total is $20 ====== fails to find the variable `$20` and raises an error. If there had been no variable error, the interpreter would have invoked `[set]` with too many arguments, causing `[set]` to raise an error. ====== eval [list set a $b] ====== Tcl changes the command to `eval {set a {the total is $20}}`, `[set]` receives the arguments `a {the total is $20}`, and the value of `$a` becomes `the total is $20`. See [double substitution] for the details of why it can be dangerous. ** Is `eval` Evil? ** Short answer: No. Like many commands in Tcl, `eval` can lead to [double substitution]. Sometimes this is desirable, and sometimes it isn't. The skilled Tcl programmer knows how to wield [double substitution]. Historically, `eval` was often used with exec, to flatten lists, allowing a single list to provide multiple arguments to a command: ====== #warning: no-longer recommended eval exec grep foo $filelist ====== because otherwise ''grep'' would receive filelist as one long filename with embedded blanks. In modern Tcl, the following is preferred for reasons of efficiency and safety: ====== exec grep foo {*}$filelist ====== Or, if you want to append one list's elements to another, and you're willing to assume that the variable containing the new elements doesn't embed any unquoted semicolons or newlines: ====== eval lappend thislist $thatlist ====== Though the following two options are safer and faster: ====== set thislist [concat $thislist $thatlist] ;# Tcl 8.4 lappend thislist {*}$thatlist ;# Tcl 8.5+ ====== Another application is in building up a script in pieces (by appending to a string) and finally calling ====== eval $script ====== When the script contains only one command, [argument expansion] is preferred: ====== {*}$cmd ====== ** Expanding Lists with `eval` ** Prior to Tcl version 8.5, one use case for `eval` was to expand lists. There is no longer any reason to do this. Use [{*}] instead. See [Argument expansion]. ---- Instead of: ====== #warning: not recommended eval pack [winfo children .] ====== use: ====== pack {*}[winfo children .] ====== ---- And instead of: ====== #warning: this is not recommended set filepath [list path to the file] eval file join $filepath ====== use: ====== set filepath [list path to the file] file join {*}$filepath ====== ---- Another similar example ====== #the old (bad) way set f [glob *.tcl] eval [linsert $f 0 exec lp] ====== ====== #the shiny new way exec lp {*}$f ====== *** Historical *** The following describes the situation before the advent of [{*}]: Naive code might look like this (an actual example from BWidget's entry.tcl:97): ====== #warning: bad code ahead! eval entry $path $maps(:cmd) ====== An unsuccessful attempt to fix the problem: ====== #warning: bad code ahead! eval entry [list $path] $maps(:cmd) ====== But `$maps(:cmd)` might be a string with newlines: ====== set maps(:cmd) { -opt1 val1 -opt2 val2 } ====== It's necessary to first convert `$maps(:cmd)` to a proper list using one of the list commands: ====== eval [linsert $maps(:cmd) 0 entry $path] ====== *** See Also *** [http://code.activestate.com/lists/tcl-core/414/%|%RE: TCLCORE Re: CFV: TIPs #156 and #157], [Jeff Hobbs], [Tcl Core Team%|%tcl-core mailing list], 2003-10-12: [AMG]: Text repeated below, please read it until you understand what trouble [[eval]] buys you and how helpful [{*}] is. [http://web.archive.org/web/20060325121839/http://aspn.activestate.com/ASPN/Mail/Message/tcl-core/1748289%|%TCLCORE TIP #144: Argument Expansion Syntax], [Jeff Hobbs], [Tcl Core Team%|%tcl-core mailing list], 2003-07-26: [http://groups.google.com/group/comp.lang.tcl/browse_thread/thread/c3d6651005e12629/a9dc3e05d95b0f12?hl=en&ie=UTF-8&q=argument+expansion+apply+variadic+group:comp.lang.tcl*#%|%eval is evil (for spreading list-arguments)], [comp.lang.tcl], 2003-03-11: **** Re: CFV: TIPs #156 and #157 **** [Jeff Hobbs]: ======none > miguel sofer writes: > > In tcllib, every effort is done to provide code that runs > > conditionally on the tcl version and provides the new functionality > > toold interpreters. > > But the TIP doesn't specify any new functionality. It only > specifies a new syntax for functionality that we already > have. I don't see a need to complicate code by providing two > implementations, when one of the implementations (the old > one) works on all versions of Tcl and has no functional drawbacks. Alright, this is where I step in to note how important this change is, and to correct everyone's completely false assumption that the existing eval hell has no functional drawbacks. dgp asked me to post these points that I made at Tcl2003 earlier, but I didn't think it necessary. It obviously is. It goes a little something like this: Raise your hand if you think this is correct: eval entry $path $args Everyone raising their hand please sit down. You are wrong. The $path arg will be split apart as well, which is bad when using this in low level code, like megawidgets or the like, where it must be handled correctly. After all, widgets with spaces in the names is 100% valid. OK, so we know the fix, right? eval entry [list $path] $args Ah, that's better ... but something is not right. Hmmm ... oh, it is inefficient! The mix of list and string args will walk the wrong path for optimization (this isn't important to everybody, but good low level code writers should be sensitive to this). OK, so that means this is the best, right? eval [list entry $path] $args Now I feel better. What, that's not right? If string args is actually a multiline string, it won't work as expected. Try it with: set args { -opt1 val1 -opt2 val2 } and unfortunately that isn't theoretical. I've seen code that uses a $defaultArgs set up like that before regular args to handle defaults. Ugh ... what are we left with? This: eval [linsert $args 0 entry $path] Only the final version is 100% correct, guaranteed not to blow when you least want it to. So we get back to the original point ... eval itself may not be functionally flawed, but 99% of eval uses are. In fact I don't always use the final solution in code because it can get so unwieldly, but now let me focus on tcllib, which was mentioned. First let me say that my favored solution is so much easier to use, has a minimal ugly factor, and doesn't have any of the flaws above: entry $path {*}$args So on to tcllib. I just grep the .tcl files for "eval" and let me pick a few: # I sure hope critcl isn't in a dir with a space ./sak.tcl: eval exec $critcl -force \ -libdir [list $target] -pkg [list $pkg] $files # Isn't this beautifully easy to understand? ./modules/ftp/ftp.tcl: eval [concat $ftp(Output) {$s $msg $state}] # I sure hope they don't use namespaces with spaces, or cmds ... ./modules/irc/irc.tcl: eval [namespace current]::cmd-$cmd $args # hmmm, just looks dangerous ... ./modules/struct/record.tcl: eval Create $def ${inst_}.${inst} \ [lindex $args $cnt_plus] # I'm not sure why eval was used here. # I think because version can be empty? ./modules/stooop/mkpkgidx.tcl: eval package require $name $version # I think someone needs to look up "concat" ./modules/textutil/adjust.tcl: lappend list [ eval list $i $words($i) 1 ] OK ... so I'm tired of looking now. Jeff Hobbs The Tcl Guy Senior Developer http://www.ActiveState.com/ Tcl Support and Productivity Solutions ====== ** Caveat: List-Like Strings ** The arguments to `eval` are concatenated into a string to be interpreted, but this operation does not guarantee that the string will be a well-formed script (i.e., one conforming to the Tcl parsing rules as laid out in the Tcl manual page). The following script breaks because the concatenation keeps the newlines from the list's string representation, making `eval` interpret the second element as a new command: ======none % set arg {a b c } a b c % eval list $arg ambiguous command name "b": bgerror binary break ====== To solve this, construct the argument using list primitives like `[lappend]`, `[list]`, etc. DKF says: "list and eval are truly made for each other." Another solution is to use the following idiom: ======none % eval [linsert $arg 0 list] a b c ====== `[linsert]` converts its list argument to a well-formed list with single spaces separating elements, and then inserts further elements into it (if any of these elements contain newlines, they remain in the resulting string). It's important to remember that `eval` works on '''strings''', not lists, and the rules for interpreting a string as a list are different than the rules for interpreting a string as a script. ** Verbose Evaluation ** [LV]: I just had to copy this over to the wiki - it is so neat! From comp.lang.tcl, [Bob Techentin] writes in response to a poster: It sounds like you're looking for something similar to /bin/sh "set -v" command which prints shell input lines as they are read, and "set -x" which prints expanded commands. Kind of like a verbose mode. Nope. Nothing like that. But you wouldn't have to write your own shell. You could walk through a script (a list of lines), and use `[info complete]` to decide how many lines are required to complete the command, then print both the command and the command's results. This seems to work. ====== proc verbose_eval {script} { set cmd {} foreach line [split $script \n] { if {$line eq {}} {continue} append cmd $line\n if {[info complete $cmd]} { puts -nonewline $cmd puts -nonewline [uplevel 1 $cmd] set cmd "" } } } set script { puts hello expr {2.2 * 3} } verbose_eval $script ====== [LV]: This proc is so slick! I love it! [Lars H]: Another approach to this (which also can see individual commands in `[if]` branches, loop bodies, etc.) is to use `[trace]`s. Do we have that written down anywhere? [AR] Here is my proposal with traces. Rem: the "noop" string is some kind of magic cookie. ** Evaluating Code in a Scoped Manner ** [wtracy] 2008-06-23: I want the commands I run in eval to have their own set of variables independently of the calling script. Is there some way I can hand eval a context (maybe as an associative array?). [Lars H]: Sounds like you want a [lambda] (or possibly a [closure], which is more difficult). If you're using Tcl 8.5, then have a look at `[apply]`. [RS]: Long before 8.5, you could always write a little proc to hide its local variables (x in this example): ======none % eval {proc {} x {expr {$x*$x}}; {} 5} 25 ====== [wtracy]: Thanks. It looks like `[apply]` is the closest Tcl feature to what I want. [wtracy]: Actually, it looks like what I *really* want is to nest `eval` inside of a `[namespace eval]` block. [wtracy]: Okay, for the sake of any lost soul that comes along after me, this is what I really wanted to do all along: ======none $ set cmd {puts FOO} puts FOO $ namespace eval MyNameSpace $cmd FOO ====== [NEM]: As mentioned, `[apply]` is probably a better choice in this case as it is less surprising wrt. variables. See [dangers of creative writing] for some related discussion. The `[apply]` version is: ====== apply {{} {puts FOO} ::MyNameSpace} ====== This has the benefit of evaluating the code within a fresh procedure context, meaning all variables are local to that context by default. See also `[namespace inscope]`/`[namespace code]` and you may also like `[dict with]`. ** Proposal: Modify `eval` to Accept a List of Lists ** In some cases `eval` does work on lists - and its special. In particular, if `eval` is passed a [pure list] then it gets evaluated directly, without another round of substitution. However, I think the utility is just short of what it ''could'' be. When you pass eval a pure list, you can only execute a single command. What if we were to pass eval a list of pure lists - it should directly evaluate each of them, and return the value of the the last one evaluated. This sounds a lot like progn in [lisp], and it seems like it would allow for some nifty bits of [introspection] - for example, if `[info body]` returned a pure list of lists rather than a string, that could be evaluated directly. Or modified. The whole program becomes a list of lists. The problem is how to signal to `eval` (or `[uplevel]`, `[namespace]`, `[bind]`, ...) that it is a list of lists rather than just a list. Could `eval` determine if its input is a pure list and the first argument of that is a pure list, then evaluate as progn? Another place this seems like it could be useful: I have a [pkgIndex.tcl] file with the line ====== package ifneeded tls 1.4 [ list load [file join $dir libtls1.4.so]];[ list source [file join $dir tls.tcl]] ====== where most of the lines are like ====== package ifneeded tclperl 2.3 [list load [file join $dir tclperl.so]] ====== since they only need one command; but the tls line needs two commands. So why can't it be ====== package ifneeded tls 1.4 [list [ list load [file join $dir libtls1.4.so]] [ list source [file join $dir tls.tcl]]] ====== ** Progn ** [RS] 2004-02-06: As usual in Tcl, functionality you miss you can easy roll yourself. I needed a progn-like list eval in [RPN again], and did it similar to this: ====== proc leval args { foreach arg $args { set res [uplevel 1 $arg] } set res ;# return last ("n-th") result, as Tcl evaluation does } ====== ** `eval` versus bytecode ** [AMG]: It appears `eval`'ed code does not get [bytecode]-compiled, even when `eval` is passed a single brace-quoted argument. The same is true for `[uplevel] 0` and `[time]`. `[catch]` and `[if] 1` seem to be bytecoded. [{*}] also gets bytecoded, but it doesn't accept a script "argument", rather a single command line pre-chewed into an objv list. Bytecoding cannot take place when the argument is the product of `[list]`, since the script isn't in a permanent [Tcl_Obj]. ====== proc a {} {eval {for {set i 0} {$i < 100000} {incr i} {}}} proc b {} {uplevel 0 {for {set i 0} {$i < 100000} {incr i} {}}} proc c {} {time {for {set i 0} {$i < 100000} {incr i} {}}} proc d {} {catch {for {set i 0} {$i < 100000} {incr i} {}}} proc e {} {if {1} {for {set i 0} {$i < 100000} {incr i} {}}} proc f {} { {*}{for {set i 0} {$i < 100000} {incr i} {}}} proc g {} {eval [list for {set i 0} {$i < 100000} {incr i} {}]} proc h {} {uplevel 0 [list for {set i 0} {$i < 100000} {incr i} {}]} proc i {} {time [list for {set i 0} {$i < 100000} {incr i} {}]} proc j {} {catch [list for {set i 0} {$i < 100000} {incr i} {}]} proc k {} {if {1} [list for {set i 0} {$i < 100000} {incr i} {}]} proc l {} { {*}[list for {set i 0} {$i < 100000} {incr i} {}]} proc m {} {try {for {set i 0} {$i < 100000} {incr i} {}}} a;b;c;d;e;f;g;h;i;j;k;l;m time a 10 ;# 80723.8 microseconds per iteration - slow time b 10 ;# 65380.2 microseconds per iteration - slow time c 10 ;# 66024.8 microseconds per iteration - slow time d 100 ;# 18888.3 microseconds per iteration - fast time e 100 ;# 18779.3 microseconds per iteration - fast time f 100 ;# 19375.2 microseconds per iteration - fast time g 10 ;# 319111.5 microseconds per iteration - very slow time h 10 ;# 342878.4 microseconds per iteration - very slow time i 10 ;# 322279.2 microseconds per iteration - very slow time j 10 ;# 316939.0 microseconds per iteration - very slow time k 10 ;# 321865.5 microseconds per iteration - very slow time l 10 ;# 344009.5 microseconds per iteration - very slow time m 100 ;# 19503.0 microseconds per iteration - fast ====== I want single-argument `eval` functionality in my code, but I also want bytecoding. `[catch]` has the undesirable side effect of hiding errors, so I guess I have to use `[if] 1 ...` which is a really weird idiom. Does anyone have any better ideas? [AMG]: I reran the tests and got better numbers. The current version of Tcl must be faster than whatever I used when I first did this benchmark (I'm using the same computer). Look in the page history to see the comparison. More importantly, I think I found a bytecoded `eval`: single-argument `[try]`. I added a test for it, and it turns out to be just as fast as the other "fast" methods. It's considerably less weird than `[if] 1`, and it doesn't hide errors. [DKF]: We've been focusing a bit more on improving the most cripplingly-slow cases, but three execution modes still exist that have fundamentally different speeds. There's compilation to bytecode-with-local-var-table (which is the fastest; the speed comes from being able to compile in indexes into the LVT into the generated bytecode, which this test is particularly sensitive to), there's compilation to bytecode-without-LVT (slower; variables have to be looked up each time they're accessed), and there's interpreting (slowest by far). There's not much point in doing a lot of comparison between the three; they all exist for a reason. (We could make straight `eval`/`[uplevel] 0` of constant arguments work at full speed, but we see no reason to bother given how rare they are in real code, and it's better that `[time]` is kept simple anyway.) You can usually tell what sort of class of speed you're going to get by using `[::tcl::unsupported::disassemble]` to look at the bytecode to see whether variables are accessed by index or by name, or whether everything's done by just building arguments and invoking “cnormal” commands. [Twylite] 2012-08-24: I've been optimising some control constructs and was trying to understand the performance of various combinations of uplevel, tailcall, catch and try. Along the way I found [AMG]'s performance figures, and have updated them with some new ones: ====== # Fast (prefix f) proc f_baseline {} { for {set i 0} {$i < 100000} {incr i} {} } proc f_catch {} {catch {for {set i 0} {$i < 100000} {incr i} {}}} proc f_if_1 {} {if 1 {for {set i 0} {$i < 100000} {incr i} {}}} proc f_expand {} { {*}{for {set i 0} {$i < 100000} {incr i} {}}} proc f_try {} {try {for {set i 0} {$i < 100000} {incr i} {}}} proc f_time_apply {} {time {apply {{} {for {set i 0} {$i < 100000} {incr i} {}}}}} # Medium (prefix m) proc m_eval {} {eval {for {set i 0} {$i < 100000} {incr i} {}}} proc m_uplevel_0 {} {uplevel 0 {for {set i 0} {$i < 100000} {incr i} {}}} proc m_time {} {time {for {set i 0} {$i < 100000} {incr i} {}}} proc m_tailcall_try {} {tailcall try {for {set i 0} {$i < 100000} {incr i} {}}} proc m_catch_uplvl_1 {} { set body {for {set i 0} {$i < 100000} {incr i} {}} ; catch { uplevel 1 $body } } proc m_uplvl_1_catch {} { set body {for {set i 0} {$i < 100000} {incr i} {}} ; uplevel 1 [list catch $body] } # Slow (prefix s) proc s_eval_list {} {eval [list for {set i 0} {$i < 100000} {incr i} {}]} proc s_uplevel_0_list {} {uplevel 0 [list for {set i 0} {$i < 100000} {incr i} {}]} proc s_time_list {} {time [list for {set i 0} {$i < 100000} {incr i} {}]} proc s_catch_list {} {catch [list for {set i 0} {$i < 100000} {incr i} {}]} proc s_if_1_list {} {if 1 [list for {set i 0} {$i < 100000} {incr i} {}]} proc s_expand_list {} { {*}[list for {set i 0} {$i < 100000} {incr i} {}]} proc s_tailcall {} {tailcall for {set i 0} {$i < 100000} {incr i} {} } if 0 { set REPS {f 10 m 40 s 100} ;# reps by prefix set cmds {f_baseline f_catch f_if_1 f_expand f_try f_time_apply m_eval m_uplevel_0 m_time m_tailcall_try m_catch_uplvl_1 m_uplvl_1_catch s_eval_list s_uplevel_0_list s_time_list s_catch_list s_if_1_list s_expand_list s_tailcall} foreach cmd $cmds {$cmd} ;# compile set cmdrepstimes {} ; foreach cmd $cmds { ;# time set reps [dict get $::REPS [string index $cmd 0]] lappend cmdrepstimes [lindex [time $cmd $reps] 0] $cmd $reps } set mintime [::tcl::mathfunc::min {*}$times] foreach {t cmd reps} [lsort -real -stride 3 $cmdrepstimes] { puts [format "time %-16s $reps\t;# %9.1f microseconds per iteration, factor %5.2f" \ $cmd $t [expr { $t / $mintime }] ] } } time f_time_apply 10 ;# 4625.8 microseconds per iteration, factor 1.00 time f_if_1 10 ;# 4641.8 microseconds per iteration, factor 1.00 time f_expand 10 ;# 4645.0 microseconds per iteration, factor 1.00 time f_catch 10 ;# 4651.3 microseconds per iteration, factor 1.00 time f_baseline 10 ;# 4658.5 microseconds per iteration, factor 1.01 time f_try 10 ;# 4735.9 microseconds per iteration, factor 1.02 time m_eval 40 ;# 18454.3 microseconds per iteration, factor 3.98 time m_uplevel_0 40 ;# 18738.3 microseconds per iteration, factor 4.04 time m_time 40 ;# 19176.8 microseconds per iteration, factor 4.14 time m_uplvl_1_catch 40 ;# 21501.1 microseconds per iteration, factor 4.64 time m_catch_uplvl_1 40 ;# 21603.1 microseconds per iteration, factor 4.66 time m_tailcall_try 40 ;# 21698.4 microseconds per iteration, factor 4.68 time s_uplevel_0_list 100 ;# 84963.3 microseconds per iteration, factor 18.34 time s_eval_list 100 ;# 85519.3 microseconds per iteration, factor 18.46 time s_catch_list 100 ;# 85919.9 microseconds per iteration, factor 18.54 time s_time_list 100 ;# 85944.2 microseconds per iteration, factor 18.55 time s_if_1_list 100 ;# 87269.0 microseconds per iteration, factor 18.84 time s_expand_list 100 ;# 93096.9 microseconds per iteration, factor 20.09 time s_tailcall 100 ;# 94473.5 microseconds per iteration, factor 20.39 ====== <> Command | Tcl syntax help | Arts and crafts of Tcl-Tk programming