Tcl 9.0/Tk9.0 Documentation > Tcl Commands, version 9.0.0 > const

Tcl/Tk Applications | Tcl Commands | Tk Commands | [incr Tcl] Package Commands | SQLite3 Package Commands | TDBC Package Commands | tdbc::mysql Package Commands | tdbc::odbc Package Commands | tdbc::postgres Package Commands | tdbc::sqlite3 Package Commands | Thread Package Commands | Tcl C API | Tk C API | [incr Tcl] Package C API | TDBC Package C API

NAME

const — create and initialize a constant

SYNOPSIS

const varName value

DESCRIPTION

This command is normally used within a procedure body (or method body, or lambda term) to create a constant within that procedure, or within a namespace eval body to create a constant within that namespace. The constant is an unmodifiable variable, called varName, that is initialized with value. The result of const is always the empty string on success.

If a variable varName does not exist, it is created with its value set to value and marked as a constant; this means that no other command (e.g., set, append, incr, unset) may modify or remove the variable; variables are checked for whether they are constants before any traces are called. If a variable varName already exists, it is an error unless that variable is marked as a constant (in which case const is a no-op).

The varName may not be a qualified name or reference an element of an array by any means. If the variable exists and is an array, that is an error.

Constants are normally only removed by their containing procedure exiting or their namespace being deleted.

EXAMPLES

Create a constant in a procedure:

proc foo {a b} {
    const BAR 12345
    return [expr {$a + $b + $BAR}]
}

Create a constant in a namespace to factor out a regular expression:

namespace eval someNS {
    const FOO_MATCHER {(?i)}\mfoo\M}

    proc findFoos str {
        variable FOO_MATCHER
        regexp -all $FOO_MATCHER $str
    }

    proc findFooIndices str {
        variable FOO_MATCHER
        regexp -all -indices $FOO_MATCHER $str
    }
}

Making a constant in a loop doesn't error:

proc foo {n} {
    set result {}
    for {set i 0} {$i < $n} {incr i} {
        const X 123
	lappend result [expr {$X + $i**2}]
    }
}

SEE ALSO

proc, namespace, set, unset

KEYWORDS

namespace, procedure, variable, constant
Copyright © 2023 Donal K. Fellows