'''[http://www.tcl.tk/man/tcl/TclCmd/variable.htm%|%variable]''', a [Tcl Commands%|%built-in] [Tcl] [command] since [Changes in Tcl/Tk 8.0%|%8.0], creates and optionally [set%|%sets] Commands%|%built-in] [Tcl] [command], declares variables in a [namespace]. : '''variable''' ?''name value...''? ''name'' ?''value''? [http://www.tcl.tk/man/tcl/TclCmd/variable.htm%|%official reference]: [http://www.tcl.tk/man/tcl/TclCmd/variable.htm%|%official reference documentation]: Creates a variable named by the `[namespace tail%|%tail]` of ''name'', which `variable` is normally used within `[namespace eval]` to create one or more variables within a `[namespace]`. For each ''name'', a variable is initialized with ''value''. The value for the last variable is optional. If a variable name does not exist, it is created. In this case, if ''value'' is specified, it is assigned to the newly created variable. If no ''value'' is specified, the new variable is left undefined. If the variable already exists, it is set to ''value'' if ''value'' is specified or left unchanged if no ''value'' is given. Normally, ''name'' is unqualified (does not include the names of any containing namespaces), and the variable is created in the current namespace. If ''name'' includes any namespace qualifiers, the variable is created in the specified namespace. ====== To access a namespace variable from inside a [proc]edcure, declare it using `variable`. In this way `[variable]` resembles `[global]`, although `[global]` is only used to refer to variables in the global namespace. If ''value''s are given, they are used to modify the values of the associated namespace variables. If a namespace variable does not exist, it is created and optionally initialized. ''name'' can refer to an array, but not to an element within an array. When ''name'' refers to an array, ''value'' must not be used. This implies that each array must be declared by a separate invocation of `[variable]`. After the variable has been declared, elements within the array can be set using ordinary `[set]` or `[array]` commands. After an array variable has been created, member variables `[variable]` can entirely replace `[global]`, so procedures can be put in a namespace without need for rewriting. The drawback is that you can't specify several variables in one command without assigning values, so instead of `[variable]` provides all the functionality of `[global]`. Replacing global foo bar grill ====== requires you'd have to write ====== variable foo; variable bar; variable grill ====== [RS]: But the fewer globals you use, the better anyway ;-) [RS]: But the less globals you use, the better anyway ;-) ---- idioms I am not familiar with. One of them is: ====== variable {} ====== What the heck is that supposed to ''do''? [DGP]: Just what the docs say. It creates a local variable whose name is the empty string within the procedure and links it to the namespace variable named empty string within the procedure and links it the namespace variable named `namespace current::` . Note that `[namespace tail] [[namespace current]]:: is the [null] string. In this particular case, the variable named the [empty string] is an [array] variable, so In this particular case, is an array variable, so one can set and read its elements like so: set (elem1) 1 set (elem2) 2 if {$(elem1) > $(elem2)} { set greater elem1 } else { set greater elem2 } puts "Greater is $($greater)" ====== [escargo] 2003-09-05: Gosh wow. Not only is [everything is a string], but ''nothing is a string.'' [RS]: 'Everything' includes 'nothing'... [KJN]: 2004-08-12: I came here looking for an explanation of this unusual idiom, and I'm pleased to find it! I had not realised until now that the minimum number of characters in a ''name'' is zero! (Is that mentioned in the manual anywhere?) Even without ''varName'' is zero! (Is that mentioned in the manual anywhere?) Even without ====== set {} 12 puts ${} set {} ====== This code will echo `12` to stdout, and return the value `12`. The [empty string] is an acceptable name for a [proc]: Even more unusual: the [null] string is acceptable as the name of a proc: ====== proc {} a { puts $a return $a } {} 42 ====== which echoes `42` to [stdout] and [return%|%returns] `42`. will echo `42` to stdout, and return the value `42`. [RS]: For scalar variables and commands you still have to delimit the "nothing" with quotes or braces, but with array names, nothing is enough. The Tcl documentation says: with quotes or braces, but with array names, nothing is enough. Man Tcl says: ====== $''name''(index) ====== ''name'' is the name of an array variable and ''index'' is the name of an element within that array. Name must contain only letters, digits, underscores, and namespace separators, '''and may be an [empty string]'''. Hence the and namespace separators, '''and may be an empty string'''. - Hence the ---- [Dossy] and [Helmut Giese] were recently discussing `[variable]` on Recently [Dossy] and [Helmut Giese] were discussing the `[variable]` on ======none So you have to distinguish between 'creating' a variable and 'defining' it. In your example you _create_ 'foo::bar' but don't _define_ it. Hence [info exist] doesn't see it - as told in the book. ====== ======none Ahh, yes. Okay, so then the docs and the behavior ARE consistent, cool. I didn't realize that a variable could be "created" but not "exist" -- weird. :-) ====== ====== % namespace eval foo { proc foo {} { variable myarr upvar somearr myarr parray myarr } proc foo2 {} { variable myarr namespace which -variable myarr } proc foo::foo3 {} { variable myarr info exists myarr } } % foo::foo "myarr" isn't an array % set somearr(x) y y % foo::foo myarr(x) = y % foo::foo2 ::foo::myarr % foo::foo3 0 ====== ---- ---- Currently (8.6.4), the command [KPV] 2003-09-13: My biggest problem with using `[variable]` instead of `[global]` is in [debugging]. I typically debug via a console window where I paste in code from the procedure I'm interested in. When I define my variables in as globals, it just works; but when the variables are buried in a namespace it doesn't--I either have to hand tweak the code to fully qualify the variable names or `[upvar]` them into the global namespace. <> Tcl syntax help | Arts and crafts of Tcl-Tk programming | Command | Glossary