TMG is a line based macro processor implemented in 22 SLOC. The first character of the line determines how to expand it, in particular:

1. @ - its just plain code, like a foreach loop, e.g. @foreach i {1 2 3} {
2. $ - just expand any variables using, e.g. $ and I is $i
3. ! - expand both variables and commands, e.g. ! \expr sin($i)\
4. Anything else is just a literal line

The implementation is:

<verbatim> proc tmg {s} { set prog {} set ::_out {} foreach line split $s \n { if {regexp {^[@(.*)} $line -> rest]} { append prog $rest \n } elseif {regexp {^[$(.*)} $line -> rest]} { append prog "append _out \subst -nocommands -nobackslashes [list $rest\] \\n" \n } elseif {regexp {^[!(.*)} $line -> rest]} { append prog "append _out \subst [list $rest\] \\n" \n } else { append prog "append _out list $line \\n" \n } } if {catch [list uplevel #0 $prog r]} { puts "$prog failed $r $::errorInfo" return "" } else { return $r } } </verbatim>

SafeTCL lets us run code with a limited set of commands so if we combine it TMG we can build code and data in a moderately secure way.

<verbatim> proc safetcl {code {name {}}} { set si interp create -safe if {$name != ""} { interp eval $si list set name $name } foreach c info commands {[%*=?!*}] { interp alias $si $c {} $c } foreach c {puts verbose check check_never exit} { interp alias $si $c {} $c } set r interp eval $si $code interp delete $si return $r } </verbatim>