[Richard Suchenwirth] 2002-05-05 - Static variables are seen here as variables
that are local to a procedure, but retain their value ("state") between calls to the procedure. One way to maintain a variable's state is to make it global (which includes putting it into a namespace), but this is frowned upon
as it clutters the environment. Another way, as shown in ''intgen'' in
[Braintwisters], is to store the value as default for a proc argument,
and rewrite the proc definition. To generalize the specific approach
taken there, we first write a proc that rewrites one argument's default with a given value:
 proc redefaultProc {procn argn value} {
    set argl {}
    foreach arg [info args $procn] {
        if [info default $procn $arg default] {
            if {$arg==$argn} {set default $value}
            lappend argl [list $arg $default]
        } else {
            lappend argl $arg
        }
    }
    proc $procn $argl [info body $procn]
 }
if 0 {With this tool in hand, we can proceed to write
''static'', a command that somehow resembles a C "declaration", but
actively registers arguments as "static", i.e. installs a write
trace so every time the variable is changed, the proc definition is
rewritten (for performance, don't assign to it too often!):}
 proc static args {
    set caller [lindex [info level -1] 0]
    foreach arg $args {
        uplevel 1 [list trace var $arg w [list staticTrace $caller]]
    }
 }
 # ... and the trace proc, which only serves to ignore 'el' and 'op':
 proc staticTrace {caller name el op} {
    upvar 1 $name var
    redefaultProc $caller $name $var 
 }
if 0 {..and now testing the whole thing. Note that a ''static'' variable
needs also to be written into the proc argument list with a default value to
start with:
 proc x {y {z 0}} {
    static z
    puts [incr z]:$y
 }
 % x hello
 1:hello
 % x world
 2:world
Looks like we made it - proc x has a "counter" z that remembers how
often it was called before, but you can reset it explicitly (as the call
to x pre-increments, use -1 to start next time with a 0 counter):
 % x reset -1
 0:reset
 % x test
 1:test
Static variables need not be numbers, for instance this example shows
that it always remembers the previous call's argument (any string):
 proc lookback {now {before {}}} {
     static before
     puts $before->$now
     set before $now
 }
And ''intgen'' can now be rewritten like this:
 proc intgen {{seed 0}} {static seed; incr seed}
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