proc(3tcl)                    Tcl Built-In Commands                   proc(3tcl)


       proc - Create a Tcl procedure

       proc name args body

       The proc command creates a new Tcl procedure named name, replacing any
       existing command or procedure there may have been by that name.  Whenever
       the new command is invoked, the contents of body will be executed by the
       Tcl interpreter.  Normally, name is unqualified (does not include the
       names of any containing namespaces), and the new procedure is created in
       the current namespace.  If name includes any namespace qualifiers, the
       procedure is created in the specified namespace.  Args specifies the
       formal arguments to the procedure.  It consists of a list, possibly
       empty, each of whose elements specifies one argument.  Each argument
       specifier is also a list with either one or two fields.  If there is only
       a single field in the specifier then it is the name of the argument; if
       there are two fields, then the first is the argument name and the second
       is its default value.  Arguments with default values that are followed by
       non-defaulted arguments become required arguments; enough actual
       arguments must be supplied to allow all arguments up to and including the
       last required formal argument.

       When name is invoked a local variable will be created for each of the
       formal arguments to the procedure; its value will be the value of
       corresponding argument in the invoking command or the argument's default
       value.  Actual arguments are assigned to formal arguments strictly in
       order.  Arguments with default values need not be specified in a
       procedure invocation.  However, there must be enough actual arguments for
       all the formal arguments that do not have defaults, and there must not be
       any extra actual arguments.  Arguments with default values that are
       followed by non-defaulted arguments become de-facto required arguments,
       though this may change in a future version of Tcl; portable code should
       ensure that all optional arguments come after all required arguments.

       There is one special case to permit procedures with variable numbers of
       arguments.  If the last formal argument has the name “args”, then a call
       to the procedure may contain more actual arguments than the procedure has
       formal arguments.  In this case, all of the actual arguments starting at
       the one that would be assigned to args are combined into a list (as if
       the list command had been used); this combined value is assigned to the
       local variable args.

       When body is being executed, variable names normally refer to local
       variables, which are created automatically when referenced and deleted
       when the procedure returns.  One local variable is automatically created
       for each of the procedure's arguments.  Other variables can only be
       accessed by invoking one of the global, variable, upvar or namespace
       upvar commands.  The current namespace when body is executed will be the
       namespace that the procedure's name exists in, which will be the
       namespace that it was created in unless it has been changed with rename.

       The proc command returns an empty string.  When a procedure is invoked,
       the procedure's return value is the value specified in a return command.
       If the procedure does not execute an explicit return, then its return
       value is the value of the last command executed in the procedure's body.
       If an error occurs while executing the procedure body, then the
       procedure-as-a-whole will return that same error.

       This is a procedure that takes two arguments and prints both their sum
       and their product. It also returns the string “OK” to the caller as an
       explicit result.

              proc printSumProduct {x y} {
                  set sum [expr {$x + $y}]
                  set prod [expr {$x * $y}]
                  puts "sum is $sum, product is $prod"
                  return "OK"

       This is a procedure that accepts arbitrarily many arguments and prints
       them out, one by one.

              proc printArguments args {
                  foreach arg $args {
                      puts $arg

       This procedure is a bit like the incr command, except it multiplies the
       contents of the named variable by the value, which defaults to 2:

              proc mult {varName {multiplier 2}} {
                  upvar 1 $varName var
                  set var [expr {$var * $multiplier}]

       info(3tcl), unknown(3tcl)

       argument, procedure

Tcl                                                                   proc(3tcl)