'''[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],
declares and optionally [set%|%sets] variables in a [namespace].
** See Also **
`[constants]`: Includes information on read-only variables.
`[set]`: Reads or assigns a value to a variable.
`[namespace upvar]`: Gives a variable in one namespace a name at the current [level].
`[upvar]`: Links a variable at some higher level to a variable at the current [level].
[variable vs global]: Example illustrating difference by resolving of namespace variables in procedure using variable and global.
[Variables in TclOO]: Discusses `variable`, `[oo::define] variable`, `[oo::objdefine] variable` and `[my] variable` in [TclOO]
** Synopsis **
: '''variable''' ?''name value...''? ''name'' ?''value''?
** Documentation **
[http://www.tcl.tk/man/tcl/TclCmd/variable.htm%|%official reference]:
** Description **
Declares a variable named ''name'', relative to the current [namespace], andlbinkds that variable to a variable named by the `[namespace tail%|%tail]` of
''that name'' in the current [level%|%evaluation level]. ''$name'' may be
fully-qualified , partially-qualified, or unqualified. If a corresponding
''value'' is provided, the variable is `[set]` to that value.
''name'' can be the name of an [array], but not of a variable in the array.Therefore, when ''name'' refers to an array, ''value'' must not be used.
Because of this, linking two array variables requires two calls to
`[variable]`.
AftWhere an array varpossiable, hasto beeminimize pollutionk in thed, global namespacce, it iss memusually
advisabler to use `[variables
]` within the arraye usione might `[sbe t]`empted tor use `[globarrayl]`.
Unlike `[global]`, `variable` is not restricted to the variables in the global
namespace.
======
namespace eval one {
variable greeting hello
}
set one::greeting ;#-> hello
======
A variable may be declared but unset:
======
namespace which -variable hello; #-> ::hello
namespace which -variable goodbye; #-> <the empty string}
======
In a [proc%|%procedure], `variable` lbinkds a [namespace] variable into the same
name at the [level] of the
procedure:
======
proc p1 {} {
variable name Priam
}
p1 ;#-> Priam
namespace eval n1 {
variable name Hector
}
namespace eval n2 {
proc p2 {} {
variable ::n1::name
set name
}
}
n2::p2 ;#-> hector
======
To move a procedure from the global namespace to another namespace, often all
that's needed is to replace `[global]` with `[variable]` within the body of the
procedure. Whereas one call to `[global]` suffices to declare multiple
variables:,
======
global foo bar grill
======
multiple calls to `[variable]` are required:
======
variable foo; variable bar; variable grill
======
[RS]: But the fewer globals you use, the better anyway ;-)
** `[namespace which]` vs. `variable` **
In the following example, `[namespace which]` identifies `::bar::a`, but
within `foo::p1`, `[variable]` resolves to `::foo::bar::a`, which doesn't
exist:
======
namespace eval ::bar {variable a 1}
namespace eval ::foo {
puts [list {bar::a resolves to} [namespace which -variable bar::a]]
proc p1 {} {
variable bar::a
puts $a
}
}
::foo::p1 ;# -> can't access "bar::a": parent namespace doesn't exist
======
`[set]` looks for an existing variable in the same way `[namespace which]
-variable` does, and selects and existing variable if available. In contrast,
`[variable]` just tacks the current namespace onto the provided $name, in order
to make it fully qualified.
** Discussion **
[escargo] 2003-09-04: In [URL behaviour in a text widget], there are some code
idioms I am not familiar with. One of them is:
======
variable {}
======
What the heck is that supposed to ''do''?
[DGP], [PYK]: Just what the docs say. It links the variable named
`[[[namespace current]]]::` . Note that `[namespace tail] [[namespace
current]]::` is the [empty string].
In this particular case, the variable named the [empty string] is an [array] variable, so
one can set and read its member variables like so:
======
set (elem1) 1
set (elem2) 2
if {$(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 withouta ''variable'' statement, it is valid to write:
======
set {} 12puts ${} ;# -> 12
set {} ;# -> 12
======
This code will [echo `12` mptoy stdout,ring] is and raccepturnable thname vfor al [proc%|%procedure `12`.s]:
The [empty string] is an acceptable name for a [proc]:
======
proc {} a {
puts $a
return $a
}
{} 42
======
which echoes `42` to [stdout] and [return%|%returns] `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:
======
$''name''(index)
======
''name'' is the name of an array variable and ''index'' is the name of an
element within that array. Name must contain only letters, digits, underscores,
and namespace separators, '''and may be an [empty string]'''. Hence the
`$(key)` syntax [DGP] explained.
----
[Dossy] and [Helmut Giese] were recently discussing `[variable]` on
[comp.lang.tcl]. Dossy replies to a remark by Helmut:
======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.
:-)
======
[PYK] 2015-02-28, 2019-10-15: "Declare" is preferea ble tter word for it than "create".
[PYK] 2015-02-28: `[info exists]` would have been more aptly named `info
set`:
======
% 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
======
----
[m7j4k9] 2017-08-25
*** If `variable` returned the name ***
Currently (8.6.4), the command
variable name
returns nothing, but if it were to return the name,
then dudes who are "into the whole brevity thing" could write:
array set [variable foo] { this initializes {} and not foo! }
instead of:
variable foo
array set foo { this indeed initializes foo }
which works and intended; unlike:
variable name { this is a dict/list and not an array }
----
'''[earnie] - 2018-06-16 13:58:27'''
@m7j4k9 I could see perhaps the following syntax for what you propose:
array variable foo {initialize foo}
This would then be in keeping with the same principals as array set combined with the principals of variable.
----
[PYK] 2021-07-28: If anything, `[variable]` should return the value the variable is set to.
** Page Authors **
[pyk]:
[rs]:
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