set docu(constrain) {[Richard Suchenwirth] - Here's an application of write 
traces to constrain a variable's value to either a range (numeric or string
prefix, joined with two dots) or a list of permitted values, that work on
scalars, arrays or array elements. Examples:
   constrain x 0..1023
   constrain y {yes no maybe}
   constrain z(mode) A..Z
If a constraint is violated, an error is raised with a descriptive
message:
 can't set "y": value 'yesSir' outside constraint 'yes no maybe'
As usual with traces, constraints are deleted if the variable is unset.
If you still want to set an out-of-constraint value, just catch the
assignment:
 catch {set y "definitely"}
This is because write traces fire after the assignment, so the "bad"
value is there anyway. Numeric constraints don't distinguish between int
and double values; string prefixes are just that, so "foo" is inside the
range "a..z" even if it's not a single character.

You can introspect the constraints you have set by calling
 constrain name
which returns the active constraint in the same format, or an empty
string if the variable is not constrained. Also, you can remove a
constraint by overriding it with an empty string:
 constrain name ""
}

set docu(comment1) {[KPV] -- I like the idea, and like to offer a suggestion. How about an optional argument which the value the variable should take if it is out of range. Sometimes I think that throwing an error is too harsh, and catching that error still leaves that bad value in place.
}

 proc constrain {var {cond -}} {
    if {$cond=="-"} {
        set trace [lindex [lindex [uplevel 1 trace vinfo $var] end] end]
        return [join [lrange $trace 1 end] ..]
    }
    foreach i [uplevel 1 trace vinfo $var] {
        if [regexp constrain $i] {uplevel 1 trace vdelete $var $i} 
    }
    if {$cond==""} return
	if [regexp {(.+)[.][.](.+)} $cond -> from to] {
		set trace [list constrainRange $from $to]
	}  else {
	    set trace [list constrainList $cond]
	}
	uplevel 1 trace var $var w [list $trace]
 }
 proc constrainRange {from to _var el op} {
	set name $_var[expr {$el!=""? "($el)": ""}]
	upvar 1 $name var
	if {$var<$from || $var>$to} {
		return -code error "value $var outside constraint $from..$to"
	}
 }
 proc constrainList {list _var el op} {
	set name $_var[expr {$el!=""? "($el)": ""}]
	upvar 1 $name var
	if {[lsearch $list $var]<0} {
		return -code error "value '$var' outside constraint '$list'"
	}
 }

See also [constants] for traces used to prevent a variable value from being changed, and [type checking].
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[Category Concept] | [Arts and crafts of Tcl-Tk programming]