Wait, Facebook uses Haskell? I had no idea.
There is little information about the size of the codebase though. I could only dig up a talk from 2009 and a tool to modify a PHP codebase with abstract-syntax-tree transformers in Haskell [2]. Can somebody give some more insight on their internal use of the language?<p>Anyway I think it's great that they have a hybrid codebase. With projects like HipHop and this Haskell backend it looks like they are gradually moving away from PHP as their main language.<p>[1]: <a href="http://cufp.galois.com/2009/abstracts.html#ChristopherPiroEugeneLetuchy" rel="nofollow">http://cufp.galois.com/2009/abstracts.html#ChristopherPiroEu...</a>
[2]: <a href="https://github.com/facebook/lex-pass/tree/master" rel="nofollow">https://github.com/facebook/lex-pass/tree/master</a>
I'm sort of surprised that Facebook's "friends of" is a directed graph, when the actual social graph is undirected. I guess they use enough directed edges for other things in that graph, but I'm sort of surprised their social graph isn't undirected and ridiculously optimised as its own separate thing.