I believe the Erlang community is not envious of Node.js as it does non-blocking I/O natively and has ways to scale deployments easily to more than one processor (something not even built-in in Node.js). More details at http://journal.dedasys.com/2010/04/29/erlang-vs-node-js and Node.js or Erlang
What about Haskell? Can Haskell provide some of the benefits of Node.js, namely a clean solution to avoid blocking I/O without having recourse to multi-thread programming?
There are many things that are attractive with Node.js
Ok, so having watched a little of the node.js presentation that @gawi pointed me at, I can say a bit more about how Haskell compares to node.js. In the presentation, Ryan describes some of the benefits of Green Threads, but then goes on to say that he doesn't find the lack of a thread abstraction to be a disadvantage. I'd disagree with his position, particularly in the context of Haskell: I think the abstractions that threads provide are essential for making server code easier to get right, and more robust. In particular:
using one thread per connection lets you write code that expresses the communication with a single client, rather that writing code that deals with all the clients at the same time. Think of it like this: a server that handles multiple clients with threads looks almost the same as one that handles a single client; the main difference is there's a
fork somewhere in the former. If the protocol you're implementing is at all complex, managing the state machine for multiple clients simultaneously gets quite tricky, whereas threads let you just script the communication with a single client. The code is easier to get right, and easier to understand and maintain.
callbacks on a single OS thread is cooperative multitasking, as opposed to preemptive multitasking, which is what you get with threads. The main disadvantage with cooperative multitasking is that the programmer is responsible for making sure that there's no starvation. It loses modularity: make a mistake in one place, and it can screw up the whole system. This is really something you don't want to have to worry about, and preemption is the simple solution. Moreover, communication between callbacks isn't possible (it would deadlock).
concurrency isn't hard in Haskell, because most code is pure and so is thread-safe by construction. There are simple communication primitives. It's much harder to shoot yourself in the foot with concurrency in Haskell than in a language with unrestricted side effects.
Can Haskell provide some of the benefits of Node.js, namely a clean solution to avoid blocking I/O without having recourse to multi-thread programming?
Yes, in fact events and threads are unified in Haskell.
Threads are actually implemented in terms of events, and run across multiple cores, with seamless thread migration, with documented performance, and applications.
Concurrent collections nbody on 32 cores
In Haskell you have both events and threads, and as it is all events under the hood.
Read the paper describing the implementation.