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

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NAME

       apply - Apply an anonymous function

SYNOPSIS

       apply func ?arg1 arg2 ...?
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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  com-
       mand, 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 val-
       ues of the actual parameters arg1 arg2 ....  When body is being executed, 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
       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(n), uplevel(n)

KEYWORDS

       argument, procedure, anonymous function

Tcl																	  apply(n)