talkingCode

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.