Parser performance: 10% regression in 8.8

Simon Marlow marlowsd at
Thu May 9 07:35:42 UTC 2019

Thanks for bringing this up.  I've merged the PR and uploaded Happy 1.19.10
to Hackage.  Can someone else look at steps 3-5?


On Wed, 8 May 2019 at 09:51, Vladislav Zavialov <vladislav at>

> Hello ghc-devs,
> This February I did some changes to the parser that require higher rank
> types support in ‘happy’. Unfortunately, as I discovered, happy’s --coerce
> option is severely broken in the presence of higher rank types, so I had to
> disable it. My benchmarks have shown a 10% slowdown from disabling --coerce
> (
> Alongside my changes I submitted a pull request to happy which fixes the
> issue (, in the hope that it
> would get merged, released, and I could re-enable --coerce in GHC ‘happy'
> configuration.
> Unfortunately, my patch has been ignored to this day (for 3 months now),
> and the performance regression reached 8.8-alpha. We need to act swiftly if
> we want to avoid a performance regression in the actual release. Here’s
> what needs to be done:
> 1. Merge
> 2. Release a new ‘happy’
> 3. (Optional) Specify in GHC’s build system that it builds only with the
> latest 'happy' release
> 4. Restore the --coerce option in GHC’s build system ‘happy’ configuration
> 5. Backport it to the ghc-8.8 branch
> I have no access to do 1 & 2, I believe Simon Marlow does. I’d appreciate
> if someone took care of 3, currently the build system does not install
> ‘happy’ and assumes a system-wide installation without checking its
> version. This means that users of all but the newly released version will
> encounter obscure error messages. We need a version check. Then I will do
> 4, as planned, and create a merge request for 5.
> All the best,
> - Vladislav
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