Lisbonacci

A small Common Lisp project to show interactive programming possibilities using Emacs and Sly (a Slime fork).

Lisbonacci

A small Common Lisp project to show interactive programming possibilities using Emacs and Sly (a Slime fork). Because of an article at Hacker News Why Lisp? and to give a short answer to the question ‘what is so special about the common Lisp REPL?’.

The goal is to present the usage of a Common Lisp REPL to people using a small program to calculate a number, so that this number and the given value form a golden ratio.

The 3 minute video at YouTube Common Lisp and Emacs (Sly) REPL example at YouTube

The video consists of the following parts:

  1. the file ./src/main.lisp is compiled and sent to the REPL (Sly, a Slime fork that for example has Stickers, a kind of special debugging breakpoint)
  2. the Lisp package (that’s like a module in other languages) lisbonacci is loaded - see files ./lisbonacci.asd and ./src/main.lisp for the package’s definition
  3. the main loop is started by calling the function main-loop. This loop asks the user to input a number, it then calculates a value that forms a golden ratio with this given number. Afterwards it asks the user if he wants to quit. The ^G you see is the function yes-or-no-p that ‘rings’ an ASCII bell before asking its question.
  4. the function fibonacci, that calculates the n-th Fibonacci number, is changed in the editor and compiled. The function is changed at the same time in the REPL, the new version of the function is called.
  5. now the main loop is changed and compiled. This time, because the main loop still runs, we have to break the program (by typing <Ctrl>-<c> <Ctrl>-<c>) and let the REPL reload the running function mainloop.

So, hot-reloading in a Common Lisp REPL is not totally different to other languages, but it is still a bit easier to do and has less limitations, like being able to change a running function.

The Golden Ratio

Lisbonacci is a program to calculate the golden ratio to a given number.

In mathematical terms:

  • let a be the given non-negative number.
  • Lisbonacci returns a number b, so that a : b == b : a + b.

To calculate the golden ratio, Lisbonacci uses the fibonacci sequence - hence the name - as the golden ratio can be approximated by the quotient of two consecutive Fibonacci numbers.

In mathematical terms:

  • let [f0, f1, f2, ... , fn, fn+1, ... ] be the Fibonacci sequence, then the quotient fn/fn+1 ~= golden ratio. See function golden-ratio in ./src/main.lisp:

      (defun golden-ratio (number)
      "Calculate the golden ratio to the given number `number`.
      Return the number which forms a golden ratio with the given number `number`."
      (* number (/ (fibonacci 32) (fibonacci 31))))

  • each Fibonacci number fn can be recursively calculated as the sum of its two predecessors. fn = fn-1 + fn-2 (for n > 1), and f0 = 0 and f1 = 1. See the recursive function fibonacci in ./src/main.lisp:

      (defun fibonacci (n)
      "Calculate the `n`th fibonacci number.
      Return the `n`th fibonacci number"
      (if (eql n 0)
          0
          (if (eql n 1)
              1
              (+ (fibonacci (- n 1)) (fibonacci (- n 2))))))

Links:

Remarks

This is to show off the Common Lisp style REPL to users who don’t know Lisp:

  • The Lisp code is deliberately simple and uses a naive, straightforward slow implementation of the fibonacci sequence and golden ratio

To speed up the calculation of the golden ratio you should at least save the Fibonacci sequence to a list, so you don’t compute each Fibonacci number twice in the function golden-ratio in ./src/main.lisp:

(defun golden-ratio (number)
  "Calculate the golden ratio to the given number `number`.
Return the number which forms a golden ratio with the given number `number`."
  (* number (/ (fibonacci 32) (fibonacci 31))))

The function calls (fibonacci 32) and (fibonacci 31) should return elements of a list of Fibonacci numbers to avoid unnecessary computations.

License

The source code of Lisbonacci Sources at GitHub is licensed under the GPL version 3.0 and newer, see file LICENSE