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apply(3tcl) Tcl Built-In Commands 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


anonymous function, argument, lambda, procedure,







  

    
    Tcl