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apply(n)                     Tcl Built-In Commands                    apply(n)


       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  names-
       pace  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(n), uplevel(n)

       argument, procedure, anonymous function

Tcl                                                                   apply(n)