How do you create constants in Tcl programs? There are, of course, several ways. ---- [LV]: Well, one way to create a ''constant'' in a Tcl program is to just code it. For instance, in the line of code: set a "abc" The string "abc" is a constant. Note, in fact, that the string need not be in quotes: set b abc also involves a constant string "abc". But what about numeric constants? Well, a decimal value can be set thusly: set c 10 or set d "10" and, when $c or $d are used in Tcl commands which expect numeric values, the value of the variable will be treated as the number '''10'''. Note that Tcl versions up through 8.4.1 treat a leading 0 as indicating that the numeric value is octal: set e "010" does not result in $e being equal to $d; instead, it is treated as being two less than $d. Are there ways to express numeric constants in other bases, such as binary or hexidecimal? $ tclsh % set a 0xbad 0xbad % incr a 2990 % So it appears that, with appropriate notation, one can get hexidecimal. For binary, one needs to do this: proc fromBinary { digitString } { set r 0 foreach d [split $digitString {}] { incr r $r incr r $d } return $r } puts [fromBinary 0101001010101010] puts [format %x [fromBinary 0101001010101010]] or proc bits2int {bits} { #returns integer equivalent of a bitlist set bits [format %032s [join $bits {}]] binary scan [binary format B* $bits] I1 x set x } ---- '''procedure constants''' Create a procedure for each constant, which returns the value. proc FLAG1 {} { return 0x0001 } You then access the constant by calling the procedure, like [[FLAG1]. This is easy to code, but gets clumsy when you have lots of constants. Another possibility is to use an array in one constant function, but this is fairly inflexible. proc CONST { key } { array set constant { FLAG1 0x001 FLAG2 0x002 PI 3.14159 } return $constant($key) } ---- '''readonly trace''' [Brent Welch] suggests using write variable [trace]s to implement readonly variables. But since the write trace fires ''after'' the variable value has changed, you need to keep a cache of the original value somewhere. [George Howlett] made a suggestion (at a Tcl conference tutorial) that read traces are your friend. He suggested a couple of very flexible procedures. (According to [Don Porter]'s c.l.t. post) % proc _constant_read_trace {val name1 name2 ops} { upvar $name1 var set var $val } % proc constant {varName value} { uplevel [list trace variable $varName r [list _constant_read_trace $value]] } % constant PI 3.14159 % set PI 3.14159 % set PI 3; # Only in Indiana :) 3 % set PI 3.14159 ---- [RS]: Slightly modified the above, so attempts to vary a constant raise an error (it probably was one ;-): proc _constant_trace {val name1 name2 ops} { upvar $name1 var if {$ops=="w"} { return -code error "constant $val may not be changed" } set var $val } proc constant {varName value} { uplevel [list trace variable $varName rw [list _constant_trace $value]] } % set PI 1.23 can't set "PI": constant 3.14159 may not be changed ---- [RS] again: This raises no error, but keeps a constant with minimal code: proc const {name value} { uplevel 1 [list set $name $value] uplevel 1 [list trace var $name w "set $name [list $value];#" ] } Simple error-raising variation: proc const {name value} { uplevel 1 [list set $name $value] uplevel 1 [list trace var $name w {error constant ;#} ] } % const x 11 % incr x can't set "x": constant ---- [RS]: From a [comp.lang.tcl] post, this variation is not about preventing changes, but to import defined "constants" in [proc] scope: interp alias {} define {} lappend ::defines proc use_defines {} { foreach {key val} $::defines {uplevel 1 [list set $key $val]} } #-- Test and demo: define PI 3.14 define e 2.781 proc try {} { use_defines return "PI=$PI, e=$e" } % try PI=3.14, e=2.781 ---- [Karl Lehenbauer], I think, gets credit for one invention of read-only variables [[cite references]]. ---- #By George Peter Staplin set ::constants [list] proc constant {name value} { global constants lappend constants $name $value } proc constproc {name argpat body} { global constants proc $name $argpat [string map $constants $body] } constant PI 3.14159 constant PROCESS [pid] constant FLAG1 2 constant FLAG2 4 constant FLAG3 8 proc & {a b} { expr {$a & $b} } constproc test {} { puts "PI PROCESS" set n 107 puts "[& $n FLAG1] [& $n FLAG2] [& $n FLAG3]" } test ---- See also [Tcl and octal numbers]. and [Binary representation of numbers].