I think the bigger question is why they're using a C++ Thrift server. Haskell is good at transforming data - they should be able to use an OS socket and do the message decoding/encoding in pure Haskell.
the bigger question is why they're using a C++ Thrift server
Because it's already there and it already works just fine.
It's so frustrating to see this sort of comment on nearly every post about a language outside the mainstream. It's a religious argument and not a productive one.
> Because it's already there and it already works just fine.
Well clearly it didn't work just fine if its threading model was interacting poorly with that of their Haskell code. If there's a mismatch between those two components then that will continue to bite them in the future, and it's legitimate to ask whether moving the boundary would be a better way to solve the problem.
That was the last tried solution, didn't work out well. And what is holding is nothing haskell specific. Is just wasting effort re implementing good broad used code at Facebook.
I think the "why not write it all in Haskell" argument is a bit spurious. Only the engineers on the team knew best the advantages and disadvantages of the various approaches. Presumably there were very good reasons to integrate it the way that they did, whether driven by architecture, performance, knowledge, library, smallest-effective-change, or other reasons. It's clearly worked out very well in the most basic sense (it does the job). And on top of that, we now have bug fixes and other improvements submitted to ghc. Seems like a win-win.
Surely a bigger question is how a site with a basis in PHP transformed to making PHP compilers, .js frameworks, Haskell, C++ etc.
They seemed to have succeeded in creating a culture of coders/hackers that were opinionated/empowered sufficiently to use their tool of choice, and to persuade others of the advantages.
This doesn't really happen in large-cap companies (though does happen to empowered teams, but even then, only somewhat - thinking finance). And speaks to advantages.
This was covered in a previous post:
"Haskell is sandwiched between two layers of C++ in Sigma. At the top, we use the C++ thrift server. In principle, Haskell can act as a thrift server, but the C++ thrift server is more mature and performant. It also supports more features. Furthermore, it can work seamlessly with the Haskell layers below because we can call into Haskell from C++. For these reasons, it made sense to use C++ for the server layer."
https://code.facebook.com/posts/745068642270222/fighting-spa...
