Thursday 31 August 2017 Functional Programming with Rillet: Roman Numerals

const of = require('./rillet.js').of;

function roman(number) {
  return of('I').cycle().take(number).join('').
    replace(/IIIII/g, 'V').
    replace(/IIII/g,  'IV').
    replace(/VV/g,    'X').
    replace(/VIV/g,   'IX').
    replace(/XXXXX/g, 'L').
    replace(/XXXX/g,  'XL').
    replace(/LL/g,    'C').
    replace(/LXL/g,   'XC').
    replace(/CCCCC/g, 'D').
    replace(/CCCC/g,  'CD').
    replace(/DD/g,    'M').
    replace(/DCD/g,   'CM');
} // roman

Here's another little bit of functional programming silliness that, again, I've adapted from some Clojure I found in a tweet -

@KevlinHenney :) pic.twitter.com/sTwDWEPFBa

— Igal Tabachnik (@hmemcpy) August 15, 2017

I mentioned before that Lisps are inside out which, of course, this example is almost designed to show off. My JavaScript version is, I submit, undeniably easier to read and understand.

Presentational issues aside, this way of calculating Roman numerals is new to me and I love it. I was given a close cousin of this problem, given a cash amount find the smallest number of notes and coins needed, as coursework for my CSE in Computer Studies back in 1984. I solved that using repeated subtractions, and that's probably where I'd start today. This approach, treating it as a string substition problem, while not necessarily "efficient", is just beautiful.

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Tagged code, rillet, javascript, and fp

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