Recently in a Stack Builders coding dojo, we practiced an exercise suggested by Corey Haines. In this exercise, you substitute any text surrounded by '$' signs with a dictionary substitution.
At the time we did this exercise in Ruby, and I came up with an extremely imperative algorithm. Later, I decided that I wanted to see how it felt to implement in Haskell. I came up with a purely functional, type-safe implementation using the powerful Parsec library in about the same amount of lines of code as the Ruby implementation that I wrote.
Compared to the Ruby implementation which is heavily laden with conditionals, and which has a significant amount of state to track, I think that the structure of the language is much more apparent in the Haskell version. Using Parsec, this was incredibly easy to achieve (once I got a better grasp of the way that Parsec works) and I think errors would be much easier to detect in the Haskell version than in the ruby code.
In both implementations I avoided using regular expressions. I agree with those who say that we should spend less time practicing regular expressions, and more time practicing writing parsers, so that's the approach that I took in these exercises.
In Corey's talk, he says,
Take a half an hour a day, and practice the fundamentals of the language you're in. Practice using iterator methods. Practice using map. Practice using inject. Practice using some of these things so that when the time comes, you don't have to fall back on old ways.
This is certainly true, and it's interesting to also try implementing familiar algorithms in other languages to see how it makes you think about the task at hand. In this case, I think that the added safety of types, as well as the increased transparency of the language from Parsec is a big improvement.
Here's how the two algorithms look, side-by-side. What do you think?
You must first cabal install parsec if you don't already have it. Then, you can execute runghc DictionaryReplacer.hs from the command line.
MIT
Justin Leitgeb, Twitter: @justinleitgeb