proc(n) Tcl Built-In Commands proc(n)
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 con-
tents 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 cre-
ated 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 proce-
dure. It consists of a list, possibly empty, each of whose elements specifies one argu-
ment. 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 com-
mand 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 argu-
ments. 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
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 vari-
ables 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 proce-
dure-as-a-whole will return that same error.