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

[Viktor Dukhovni]

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.