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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 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 ref-erenced referenced
       erenced  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(n), uplevel(n)

KEYWORDS
       argument, procedure, anonymous function



Tcl                                                                                                 apply(n)