apply - Apply an anonymous function

       The command apply applies the function func to the arguments arg1arg2... and returns the result.

       The  function func is a two element list {argsbody} or a three element list {argsbodynamespace} (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  arg1arg2....   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 approximately this:

              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:a2:bb3:ccc4:dddd
              map {x {expr {$x**2 + 3*$x - 2}}} {-4 -3 -2 -1 0 1 2 3 4}
                    →2-2-4-4-2281626

       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

       anonymous function, argument, lambda, procedure

Tcl                                                                                                  apply(3tcl)

       apply - Apply an anonymous function

proc(3tcl), uplevel(3tcl)

applyfunc ?arg1arg2...?
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