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

[Ryan Reich]

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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