Implement main server loop in Haskell?

Question

What is the generally accepted way to implement the main loop of a server that needs to wait on a heterogeneous set of events? That is the server should wait (not busywait) until one of the following occurs:

new socket connection
data available on an existing socket
OS signal
third-party library callbacks

NovaDenizen · Accepted Answer · 2015-06-17 16:02:10Z

6

I think you're thinking in terms of a C paradigm with a single thread, nonblocking I/O, and a select() call.

You can manage to write something like that in Haskell, but Haskell has much more to offer:

lightweight threads
safe and efficient concurrent data primitives like Mvar and Chan
the Big Gun: Software Transactional Memory

I recommend you fork a new thread for every separate point of contact with the outside world, and keep everything coordinated with STM.

edited Jun 17, 2015 at 16:02

answered Sep 13, 2012 at 19:43

NovaDenizen

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Comments

Satvik · Accepted Answer · 2012-09-13 19:17:17Z

2

Use takeMVar and putMVar to synchronize between threads. They generally block the thread if operation is not permitted. Read ghc docs.

answered Sep 13, 2012 at 19:17

Satvik

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1 Comment

singpolyma Over a year ago

Or Chan. I really like Chan.

hammar · Accepted Answer · 2012-09-14 13:58:34Z

1

I'd like to make it clear I think the two solutions posted first are better than this one for the specific problem you have, but here's a way to solve the type of problem you presented.

A simple way round this is to take your definitions like

data SocketConn = ....
data DataAvail = ...
data OSSignal = ...
data Callback = ...

and define the unsimplified version of

data ServerEvent = Sok SocketConn | Dat DataAvail | Sig OSSignal | Call Callback

handleEvent :: ServerEvent -> IO ()
handleEvent (Soc s) = ....
handleEvent (Dat d) = ....
handleEvent (Sig o) = ....
handleEvent (Call c) = ....

Like I say, read up on the other answers!

edited Sep 14, 2012 at 13:58

hammar

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answered Sep 14, 2012 at 2:43

AndrewC

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2 Comments

Dan Burton Over a year ago

You shouldn't decry this answer so much; it's a legitimate way to do things. And it does answer the underlying generalized question, which is, "how does one handle the possibility of different shapes of inputs that all go into the same slot?". The answer is "use a tagged union", as you suggested.

AndrewC Over a year ago

@DanBurton OK, I've toned down the "don't do this" to "the other solutions are better for your specitic problem"

MathematicalOrchid · Accepted Answer · 2012-09-14 16:19:19Z

0

Software Transactional Memory (STM) is the main way to do a multi-way wait.

However, by the looks of things, in your case you probably just want to spawn a seperate Haskell thread for each task, and let each such thread block while there's nothing happening.

You wouldn't want to create a thousand OS threads, but a thousand Haskell threads is no trouble at all.

(If these threads need to coordinate from time to time, then again, STM is probably the simplest, most reliable way to do that.)

answered Sep 14, 2012 at 16:19

MathematicalOrchid

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