It was one of those silly ideas-nights and I decided to add the ability to have blocks of code that can be omitted from the bytecode depending on a condition as a proof-of-concept. Once the parser is in, we can start working on ‘real,’ more generic sexp and AST manipulators towards something approaching Lisp’s facilities.

I figured it would be a fairly short task and for a change I was correct&emdash;and a good portion of the thanks goes to Rubinius’ already-excellent facilities: I was able to modify the sexp with the full power of Ruby. It took me a bit to figure out the best way to set this up but in the end there are literally about 11 lines of implementation code plus the specs. The end result is a block construct for a special form that allows conditionalising. Currently I only set it up to process global variables but it is trivial to add support for the rest—what may not be trivial is for the user to get other types of conditions set up without confusion over compile-time versus runtime. The invocation is pretty simple:

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# ... Rubinius.compile_if($DEBUG) { Logger.debug "Blah blah: #{some_variable}" } # ...

If $DEBUG is false at the time this section goes through the compiler, that entire line disappears from the first R to the last }. If it evaluates to true, the block contents are processed normally&emdash;but we still remove the now-extra wrapper block around it both to avoid the overhead as well as ensure that the variable scope etc. are consistent.

This should be useful for at least removing those generally-unnecessary debug statements etc.; the if $DEBUG does not cost much by itself but it adds up over time. The other easy target are platform/library/etc. specific sections that can be omitted in some cases. It is general-purpose in that the only limiting factor is the condition variable (or possibly expression in the future.)

The commit is here.