Racket has an awesome macro system and people have managed to do amazing things with it. This paper(http://www.ccs.neu.edu/scheme/pubs/scheme2007-ctf.pdf http:

Racket has an awesome macro system and people have managed to do amazing things with it. This paper(http://www.ccs.neu.edu/scheme/pubs/scheme2007-ctf.pdf http://www.ccs.neu.edu/scheme/pubs/scheme2007-ctf.pdf) demonstrates how they made an explicitly-typed dialect of the dynamically typed language Racket. Truly awesome stuffs.