12.5.30    proc

Description:   Create a Tcl procedure (e.g. a subroutine)

Syntax:         proc name args body

         

Argument:     name args body

Returns:        procedure’s return value is the value specified in a return command

See Also:       While, foreach, for,

 

Example:      

proc power {base p} {

    set result 1

    while {$p > 0} {

        set result [expr $result * $base]

        set p [expr $p - 1]

    }

    return $result

}

 

SETVAL tag1=[power 2 6]

SETVAL answr=[power [GETVAL basetag] [GETVAL pwrtag]]

SETVAL answr2=[power [GETVAL btag2] [GETVAL ptag2]]

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 Tcl interpreter will execute the contents of body. 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.

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. 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 don’t have defaults, and there must not be any extra actual 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 formals. 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. 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 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 doesn’t 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.