Unpacking coercions

Simon Peyton Jones simonpj at microsoft.com
Tue Sep 4 22:10:59 UTC 2018

Ah yes thanks.

As I suggest on that thread, I’d really like (someone) to try the “let’s make all evidence strict” idea.  I think it could be a perf win all round.

And if so, we’d get what David wants for free.


From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Ryan Scott
Sent: 04 September 2018 17:15
To: ghc-devs at haskell.org
Subject: Re: Unpacking coercions

In case this wasn't clear, the context of this discussion in this GHC proposal [1], where David is trying to work around the fact that data types with existential Coercible constraints do not support unpacking. (By "unpacking", I mean putting an {-# UNPACK #-} pragma in front of a field of that type does what you would expect.)

An example program which demonstrates the performance issue can be found here [2]. That comment concerns unboxed equality vs. boxed equality, but the same unpacking problems that affect boxed equality also affect Coercible.

Ryan S.
[1] https://github.com/ghc-proposals/ghc-proposals/pull/116<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F116&data=02%7C01%7Csimonpj%40microsoft.com%7C6503cdd836df4b6fc63808d61281ac30%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636716745484845080&sdata=fLDLZEX015AtZpM4qfXwJAo90dMk7YBLQXGYV7wcXQo%3D&reserved=0>
[2] https://github.com/ghc-proposals/ghc-proposals/pull/116#issuecomment-382346662<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F116%23issuecomment-382346662&data=02%7C01%7Csimonpj%40microsoft.com%7C6503cdd836df4b6fc63808d61281ac30%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636716745484855092&sdata=dT3NAweHgwM%2FO7Ncjn4renyajsbkjU3UEMdgSIXaoz0%3D&reserved=0>
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