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proc(n) Tcl Built-In Commands proc(n)
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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(n), unknown(n)
KEYWORDS
argument, procedure
Tcl proc(n)