Tcl 8.7/Tk8.7a5 Documentation > Tcl Commands > apply

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apply — Apply an anonymous function


apply func ?arg1 arg2 ...?


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 corresponding 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 variable 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


This shows how to make a simple general command that applies a transformation to each element 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


proc, uplevel


anonymous function, argument, lambda, procedure,
Copyright © 2006 Miguel Sofer
Copyright © 2006 Donal K. Fellows