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

       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)