David A. Wheeler's Blog

Sun, 10 Mar 2013

Readable Lisp: Sweet-expressions

I’ve used Lisp-based programming languages for decades, but while they have some nice properties, their traditional s-expression notation is not very readable. Even the original creator of Lisp did not particularly like its notation! However, this problem turns out to be surprisingly hard to solve.

After reviewing the many past failed efforts, I think I have figured out why they failed. Past solutions typically did not work because they failed to be general (the notation is independent from any underlying semantic) or homoiconic (the underlying data structure is clear from the syntax). Once I realized that, I devised (with a lot of help from others!) a new notation, called sweet-expressions (t-expressions), that is general and homoiconic. I think this creates a real solution for an old problem.

You can download and try out sweet-expressions as released by the Readable Lisp S-expressions Project by downloading our new version 0.7.0 release.

If you’re interested, please participate! In particular, please participate in the SRFI-110 sweet-expressions (t-expressions) mailing list. SRFIs let people write specifications for extensions to the Scheme programming language (a Lisp), and this SRFI lets people in the Scheme community discuss it.

The following table shows what an example of traditional (ugly) Lisp s-expressions, the same thing in sweet-expressions, and a short explanation.

s-expressions Sweet-expressions (t-expressions) Explanation
(define (fibfast n)
  (if (< n 2)
    (fibup n 2 1 0)))
define fibfast(n)
  if {n < 2}
    fibup n 2 1 0
Typical function notation
Indentation, infix {...}
Single expr = no new list
Simple function calls

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