proc(n) Tcl Built-In Commands proc(n) ______________________________________________________________________________
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. When-
ever 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 val-
ues that are followed by non-defaulted arguments become required argu-
ments; 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 cor-
responding 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 proce-
dure 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 proce-
dure 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 cre-
ated 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 com-
mand. If the procedure does not execute an explicit return, then its
return value is the value of the last command executed in the proce-
dure'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)
tcl 8.6.6 - Generated Mon Aug 22 18:17:58 CDT 2016