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man pages section 1: User Commands

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Updated: Wednesday, February 9, 2022

apply (1t)


apply - Apply an anonymous function


apply func ?arg1 arg2 ...?


apply(1t)                    Tcl Built-In Commands                   apply(1t)


       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 specifi-
       cation  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 exe-
       cuted, variable names normally refer to local variables, which are cre-
       ated 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 pro-
       ceeds in this call frame, in the namespace given by namespace or in the
       global  namespace  if none was specified. If given, namespace is inter-
       preted 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 trans-
       formation 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 attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

       proc(n), uplevel(n)

       anonymous function, argument, lambda, procedure,

       Source  code  for open source software components in Oracle Solaris can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This     software     was    built    from    source    available    at
       https://github.com/oracle/solaris-userland.   The  original   community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                                                  apply(1t)