Richard Suchenwirth 2006-04-26 - A question on comp.lang.tcl brought me to experiment with code that rewrites itself. The idea is that settings are not saved in a separate resource file, but in the script itself, as setting a global variable with the desired value. So the next time the script is run, the previously saved setting applies. Here's what I did:
proc save {filename varlist} {
set f [open $filename]
set data [read $f]
close $f
set lines {}
foreach line [split $data \n] {
foreach var $varlist {
if [regexp "^ *set $var " $line] {
set line [list set $var [set ::$var]]
}
}
lappend lines $line
}
set f [open $filename w]
puts $f [join $lines \n]
close $f
}#-- Now for a testing demo:
package require Tk
#-- We will test with this variable:
set foo hello label .1 -text foo: entry .2 -textvar foo button .3 -text Save -command [list save [info script] foo] eval pack [winfo children .] -side left
#-- Invaluable little helper for rapid restart:
bind . <Escape> {exec wish $argv0 &; exit}lexfiend 2006-04-27 - Here's a modification that places all the state variables in a single section, so that save doesn't inadvertantly overwrite other sets of the state variables and thereby change the program logic. However, note that this forces you to pre-initialize any namespaces on which the state variables depend.
# Create empty namespaces first
namespace eval testns {}
### VARIABLE SAVE SECTION
set test1 "I am here"
set testns::test2 "Here as well"
### VARIABLE SAVE SECTION
proc save {filename varlist} {
set f [open $filename]
set data [read $f]
close $f
set lines {}
set inSaveSection 0
foreach line [split $data \n] {
if [regexp "^ *#+ *VARIABLE SAVE SECTION" $line] {
if {!$inSaveSection} {
# Start of save section
lappend lines "### VARIABLE SAVE SECTION"
foreach var $varlist {
lappend lines [list set $var [set ::$var]]
}
set inSaveSection 1
} else {
# End of save section
lappend lines "### VARIABLE SAVE SECTION"
set inSaveSection 0
}
} elseif {$inSaveSection} {
# Somewhere in save section - do nothing
} else {
lappend lines $line
}
}
set f [open $filename w]
puts $f [join $lines \n]
close $f
}
set testns::test2 "Gone forever"
set test1 "Not here now"
save [info script] {test1 testns::test2}EMJ - 2006-04-26 - Aaaaargh! Self-modifying code, the slippery slope to certain unmaintainability! That of course is a remark from the distant past, when clever people wrote very fast but generally incomprehensible self-modifying assembly language. The warning is still valid, however - be careful!
For the stated purpose, it's not actually too bad, but I think I would use an alias or something rather than just set, in order to make the regex more discriminating. I guess it's simply the reverse of what tkbiff does, writing most of its code to its config file!
IL - This kind of stuff always makes me think of DNA, the assembly of the organic life. I suppose that makes tRNA the compiler and proteins the executable... heh.