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apply(n) Tcl Built-In Commands apply(n)
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NAME
apply - Apply an anonymous function
SYNOPSIS
apply func ?arg1 arg2 ...?
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DESCRIPTION
The command apply applies the function func to the arguments arg1 arg2 ... and returns the
result.
The function func is a two element list {args body} or a three element list {args body
namespace} (as if the list command had been used). The first element args specifies the
formal arguments to func. The specification of the formal arguments args is shared with
the proc command, and is described in detail in the corresponding manual page.
The contents of body are executed by the Tcl interpreter after the local variables corre-
sponding to the formal arguments are given the values of the actual parameters arg1 arg2
.... When body is being executed, variable names normally refer to local variables, which
are created automatically when referenced and deleted when apply returns. One local vari-
able is automatically created for each of the function's arguments. Global variables can
only be accessed by invoking the global command or the upvar command. Namespace variables
can only be accessed by invoking the variable command or the upvar command.
The invocation of apply adds a call frame to Tcl's evaluation stack (the stack of frames
accessed via uplevel). The execution of body proceeds in this call frame, in the namespace
given by namespace or in the global namespace if none was specified. If given, namespace
is interpreted relative to the global namespace even if its name does not start with "::".
The semantics of apply can also be described by:
proc apply {fun args} {
set len [llength $fun]
if {($len < 2) || ($len > 3)} {
error "can't interpret \"$fun\" as anonymous function"
}
lassign $fun argList body ns
set name ::$ns::[getGloballyUniqueName]
set body0 {
rename [lindex [info level 0] 0] {}
}
proc $name $argList ${body0}$body
set code [catch {uplevel 1 $name $args} res opt]
return -options $opt $res
}
EXAMPLES
This shows how to make a simple general command that applies a transformation to each ele-
ment of a list.
proc map {lambda list} {
set result {}
foreach item $list {
lappend result [apply $lambda $item]
}
return $result
}
map {x {return [string length $x]:$x}} {a bb ccc dddd}
-> 1:a 2:bb 3:ccc 4:dddd
map {x {expr {$x**2 + 3*$x - 2}}} {-4 -3 -2 -1 0 1 2 3 4}
-> 2 -2 -4 -4 -2 2 8 16 26
The apply command is also useful for defining callbacks for use in the trace command:
set vbl "123abc"
trace add variable vbl write {apply {{v1 v2 op} {
upvar 1 $v1 v
puts "updated variable to \"$v\""
}}}
set vbl 123
set vbl abc
SEE ALSO
proc(n), uplevel(n)
KEYWORDS
argument, procedure, anonymous function
Tcl apply(n)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/2.4.6 (CentOS)
Under GNU General Public License
2025-06-23 22:12 @127.0.0.1 CrawledBy Wget/1.21.2