NAME

       proc - Create a Tcl procedure

SYNOPSIS

       proc name args body
_________________________________________________________________________________________________

DESCRIPTION

       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.

EXAMPLES

       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}]
              }

SEE ALSO

       info(3tcl), unknown(3tcl)

KEYWORDS

       argument, procedure