Getting started with Haskell
All the cool kids, it seems, are using Haskell these days. Or at least, the two or three people with whom I talk about software on a regular basis. Functional programming is fun. It makes you think a little differently about the world, and serves as a decent substitute for a unicorn chaser if you’ve been writing PHP all day.
I remember almost the first snippet of code we saw on our computer science course was the following to count the number of items in a list:
count  = 0 count (x:xs) = 1 + count(xs)
which, for a guy who’d only ever really written code in BASIC before starting the course, was a bit of an eye opener. I think it was probably three or four lectures in before we had to work out the code for printing out all the permutations of a list:
> permute [1,2,3] [[1,2,3],[2,1,3],[2,3,1],[1,3,2],[3,1,2],[3,2,1]]
After much blood, sweat, and tears we’d eventually come to the realisation that:
insert x  = [[x]] insert x (y:ys) = (x:y:ys) : map (\z -> y:z) (insert x ys) permute  = [] permute (x:xs) = concat (map (insert x) (permute xs))
would suffice (where concat flattens a list and map has the usual meaning). At first glance, it all looks pretty daunting, but after a while one comes to understand what it means for the type of map to be (a -> b) -> [a] -> [b], and starts to appreciate the compact elegance of the code that’s produced in functional programming.
Ugly implementation details? You betcha…
apt-get install hugs apt-get install haskell98-tutorial cat > hello.hs <<END #!/usr/bin/runhugs +l main :: IO() main = putStr "Hello World!\n" END chmod 755 hello.hs ./hello.hs
(N.B. hugs with the ‘+l’ switch is sensitive about filenames. Non ‘.hs’ files require lines of Haskell to begin ‘>’). It’s often more comfortable to develop these things interactively. Booting into a hugs session and typing
> :load hello.hs
will import the functions defined in hello.hs into your session.
Over the coming weeks, I hope to be honing my skills and infuriating my coworkers by replacing bits of critical infrastructure with Haskell scripts. I’ll let you know how that goes.