[Crito]

Racket is somewhat unique in this respect, but it allows you to create dramatically different languages as DSLs.

Consider this example of the experimental "2d" syntax:

  #lang unstable/2d racket
  (require unstable/2d/cond)
 
  (define (same? a b)
    #2dcond
    ╔═════════════╦═══════════════════════╦═════════════╗
    ║             ║       (pair? a)       ║ (number? a) ║
    ╠═════════════╬═══════════════════════╬═════════════╣
    ║ (pair? b)   ║ (and (same? (car a)   ║     #f      ║
    ║             ║             (car b))  ║             ║
    ║             ║      (same? (cdr a)   ║             ║
    ║             ║             (cdr b))) ║             ║
    ╠═════════════╬═══════════════════════╬═════════════╣
    ║ (number? b) ║          #f           ║   (= a b)   ║
    ╚═════════════╩═══════════════════════╩═════════════╝)

Yes, that big ascii-art graph is actually part of the code, not some sort of comment. The 2d syntax allows you to create ascii art truth tables which contain in them code (in this case, in the traditional paren'd style.)

(See http://docs.racket-lang.org/unstable/2d.html for a better explanation and more examples.)

Alternatively, here is an example of typed racket using sweet expressions:

  #lang sweet-exp typed/racket

  define: fact([n : Integer]) : Integer
    if zero?(n)
       1
       {n * fact{n - 1}}

(https://github.com/takikawa/sweet-racket)*