apply(3tcl)

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

   argument, procedure, anonymous function

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