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

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? Erik [1] https://github.com/gwern/mueval/blob/09b6a7aa5a25c4115442ea2e6ae0c2db557007f... On Thu, 29 Nov 2018 at 03:41, Ryan Reich wrote:

I expected something like that. I'm all the way back in ghc-8.2.2, but I think what this really shows is that the flag is unreliable and version-dependent. Unfortunately there doesn't seem to be a precise specification of where yield points should appear, and therefore, where they might be not-omitted.

On Wed, Nov 28, 2018, 17:10 Viktor Dukhovni

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On Wed, Nov 28, 2018 at 09:39:01AM -0800, Ryan Reich wrote:

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Thanks for the suggestion, which Arjen made also. Unfortunately, it does not appear to help. See this simple program:

-- Loop.hs import Control.Exception import System.Timeout

main :: IO (Maybe Integer) main = timeout 100000 $ evaluate $ last $ repeat 0 -- end

With either GHC invocation "stack exec ghc Loop[ -- -fno-omit-yields]", running ./Loop fails to terminate (it should do so in 0.1s).

Based only on the very terse description of that flag in the User's Guide, and its name, I think the problem is simply that GHC doesn't normally *generate* yields in that loop, so there's nothing not to omit.

It times out for me with GHC 8.4.4 on FreeBSD 11.2, and "ghc -O -fno-omit-yields" and does not time out with "ghc -O":

$ cat /tmp/foo.hs import Control.Exception import System.Timeout

main :: IO (Maybe Integer) main = timeout 1000000 $ evaluate $ last $ repeat 0

$ ghc -O -fno-omit-yields /tmp/foo.hs [1 of 1] Compiling Main ( /tmp/foo.hs, /tmp/foo.o ) [Optimisation flags changed] Linking /tmp/foo ...

$ time /tmp/foo

real 0m1.033s user 0m1.025s sys 0m0.008s

$ rm /tmp/foo $ ghc -O /tmp/foo.hs [1 of 1] Compiling Main ( /tmp/foo.hs, /tmp/foo.o ) [Optimisation flags changed] Linking /tmp/foo ...

$ time /tmp/foo ^C^C

real 0m5.864s user 0m5.857s sys 0m0.000s

On MacOS X with GHC 7.10.3, it does not time out either way. Perhaps some versions of GHC don't make the timeout possible.

-- Viktor. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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