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

[Carter Schonwald]

Yeah that’s probably a better approach.

On Thu, Nov 29, 2018 at 5:16 AM Erik Hesselink <hesselink at gmail.com> 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?
>
> Erik
>
> [1]
> https://github.com/gwern/mueval/blob/09b6a7aa5a25c4115442ea2e6ae0c2db557007f8/Mueval/Parallel.hs
>
> On Thu, 29 Nov 2018 at 03:41, Ryan Reich <ryan.reich at gmail.com> 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 <ietf-dane at dukhovni.org
>> wrote:
>>
>>> On Wed, Nov 28, 2018 at 09:39:01AM -0800, Ryan Reich wrote:
>>>
>>> > 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.
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