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(3tcl), uplevel(3tcl)

KEYWORDS

       anonymous function, argument, lambda, procedure,