Proposal: System.Timeout module for base

Peter Simons simons at
Thu Feb 1 13:05:20 EST 2007

Simon Marlow writes:

 > Any other suggestions?

The fact that blocking FFI threads cannot be timed out with the
current implementation is unpleasant, and I would be happy to
lift that limitation.

I wonder, though, whether depending on unportable low-level OS
mechanisms like pthread_cancel() or pthread signals to deliver a
timeout might prove to be far more unpleasant than the limitation
we try to lift. As far as I'm concerned, the limitation that
blocking FFI threads cannot be timed out from Haskell is not that
hard. A blocking FFI call, well, blocks. In all honesty, I
wouldn't expect that to be any different. I would be far more
surprised if it turned out that my currently running C code
suddenly receives signals and is supposed to deal with that.

My preferred approach would be to agree on the _signature_ of the
combinator. We should find consensus that timeout is supposed to
look like this:

  timeout :: Int -> IO a -> IO (Maybe a)

If that is the case, then we can add the function into the
distribution now and worry about wild improvements of the
implementation later. I feel that making even an unperfect
solution available to Haskell programmers would be a significant
step forward.

Maybe this endeavor would be easier to accomplish if we rename
the function to unsafeTimeout? ;-)


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