focal (3) proc.3tcl.gz

NAME

       proc - Create a Tcl procedure

SYNOPSIS

       proc name args body
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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