CODEWITHSUNDEEP
haskell
brooks94
brooks94

Reputation: 3936

Implement main server loop in Haskell?

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:

Upvotes: 5

Views: 366

Answers (4)

NovaDenizen
NovaDenizen

Reputation: 5315

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:

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

Upvotes: 6

MathematicalOrchid
MathematicalOrchid

Reputation: 62818

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.)

Upvotes: 0

AndrewC
AndrewC

Reputation: 32455

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!

Upvotes: 1

Satvik
Satvik

Reputation: 11208

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

Upvotes: 2

Related Questions