[Haskell-cafe] Timing out a pure evaluation of an expression I did not write myself

Bertram Felgenhauer bertram.felgenhauer at googlemail.com
Wed Dec 19 11:32:19 UTC 2018

Erik Hesselink wrote:
> Hi Ryan,
> I'm not quite sure of your use case, but lambdabot/mueval do something like
> this I think. From a brief look at the source [1] it seems they compute in
> a separate thread, then in another thread, wait until the timeout then kill
> the computation thread. Would something like that work for you?

That is not the whole story; the above mechanism is subject to the same
problems as the original poster's program.

mueval actually comes with a wrapper program ("mueval", source:
"watchdog.hs") that invokes the actual interpreter ("mueval-core",
source: "main.hs") as a subprocess; both programs implement a timeout,
but the wrapper kills the interpreter if the internal timeout fails.

This machinery predates -fno-omit-yields. But it's still useful because
it also protects against blocking unsafe foreign calls and other
scenarios where the RTS can become completely unresponsive, as well
as potential holes in -fno-omit-yields.



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