bionic (3) apply.3tcl.gz

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
       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