invar.tcl

proc invariant {var {expression ""} {msg "Illegal value"}} {
    uplevel trace variable $var w \
	\[list invariant_check \{$expression\} \{$msg\}]
}

proc invariant_check {expression msg v1 v2 mode} {
    if {$expression == ""} {
        error "Cannot modify!"
    }
    if {![uplevel expr \{$expression\}]} {
        error $msg
    }
}