---
name: "apply"
section: "n"
platform: "macos"
package: "tcl"
version: "152"
release: "macOS 26.5"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/apply/n"
---

# apply(n)

Apply an anonymous function

## NAME

apply - Apply an anonymous function

## SYNOPSIS

**apply** *func* ?*arg1 arg2 ...*?

## 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 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 }

## EXAMPLES

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

## SEE ALSO

[proc(n)](/proc/n.md), [uplevel(n)](/uplevel/n.md)

## KEYWORDS

argument, procedure, anonymous function