[ghc-steering-committee] [EXTERNAL] #265: Unlifted Datatypes, recommendation: accept

[Simon Peyton Jones]

I’m in support here.  I’ve commented on the discussion thread.

Simon

From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org> On Behalf Of Simon Marlow
Sent: 02 December 2019 10:06
To: Joachim Breitner <mail at joachim-breitner.de>
Cc: ghc-steering-committee at haskell.org
Subject: [EXTERNAL] [ghc-steering-committee] #265: Unlifted Datatypes, recommendation: accept

I recommend that we accept proposal #265 (Unlifted Datatypes)

https://github.com/ghc-proposals/ghc-proposals/pull/265<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F265&data=02%7C01%7Csimonpj%40microsoft.com%7Cbebbb53672104f8804a408d7770f544a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637108780029253857&sdata=aEWC0cJAxCRJ973miRTP%2F6JtGVp%2F4FG6qqmKjjfKywE%3D&reserved=0>
https://github.com/sgraf812/ghc-proposals/blob/unlifted-data/proposals/0000-unlifted-datatypes.rst<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fsgraf812%2Fghc-proposals%2Fblob%2Funlifted-data%2Fproposals%2F0000-unlifted-datatypes.rst&data=02%7C01%7Csimonpj%40microsoft.com%7Cbebbb53672104f8804a408d7770f544a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637108780029263813&sdata=JCfBIUeVfDO0f6CElwzpmPY9xyYj7pp%2FoojID59kkCo%3D&reserved=0>

It's a fairly conservative extension: the kind TYPE 'UnliftedRep already exists with the required functionality, the only addition here is to allow user-defined types to be declared with that kind. The semantics are clear, and there already exists a prototype patch to implement it.

There are considerable performance benefits to be had for performance-critical code, for instance the containers package.

A couple of minor issues remain:

  *   Without special support, the type data unlifted Strict a = Force !a comes with an associated box, so this type isn't as useful as it could be.
  *   It isn't possible to define values of kind TYPE 'UnlifedRep at the top level, which might be a surprising restriction to the programmer. (However, there's a reasonable workaround). Relatedly, GHC cannot lift expressions of kind TYPE 'UnlifedRep to the top level in the optimiser, which can lead to surprising performance behaviour. See https://gitlab.haskell.org/ghc/ghc/issues/17521<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fissues%2F17521&data=02%7C01%7Csimonpj%40microsoft.com%7Cbebbb53672104f8804a408d7770f544a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637108780029263813&sdata=dgDbIe9AhhI%2FIsnPDPxX7EAoVVkjaKthu%2FNGcyEBOTU%3D&reserved=0>
Nevertheless, we shouldn't let the perfect be the enemy of the good, and Unlifted Datatypes is a clearly useful addition in my view.

Cheers
Simon


On Thu, 28 Nov 2019 at 10:06, Joachim Breitner <mail at joachim-breitner.de<mailto:mail at joachim-breitner.de>> wrote:
Dear Committee,

this is your secretary speaking:

Unlifed Datatypes
has been proposed by Sebastian Graf
https://github.com/ghc-proposals/ghc-proposals/pull/265<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F265&data=02%7C01%7Csimonpj%40microsoft.com%7Cbebbb53672104f8804a408d7770f544a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637108780029263813&sdata=sbtvm3eM43ihY%2FPoTeS4Sp%2BDE2AJUBSqgGOv6HymoKg%3D&reserved=0>
https://github.com/sgraf812/ghc-proposals/blob/unlifted-data/proposals/0000-unlifted-datatypes.rst<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fsgraf812%2Fghc-proposals%2Fblob%2Funlifted-data%2Fproposals%2F0000-unlifted-datatypes.rst&data=02%7C01%7Csimonpj%40microsoft.com%7Cbebbb53672104f8804a408d7770f544a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637108780029273768&sdata=PeFWonpeS4vYeanypRBMEWgLFy9CgVeue3OhHrZa7aM%3D&reserved=0>

I propose Simon Marlow as the shepherd, as the expert on low-level stuff.

Please reach consensus as described in
https://github.com/ghc-proposals/ghc-proposals#committee-process<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%23committee-process&data=02%7C01%7Csimonpj%40microsoft.com%7Cbebbb53672104f8804a408d7770f544a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637108780029273768&sdata=9V5KT%2F6kDmUNU5xMMKByR4egiSf61TMdq6yoHMviGcg%3D&reserved=0>
I suggest you make a recommendation, in a new e-mail thread with the
proposal number in the subject, about the decision, maybe point out
debatable points, and assume that anyone who stays quiet agrees with
you.

Thanks,
Joachim
--
Joachim Breitner
  mail at joachim-breitner.de<mailto:mail at joachim-breitner.de>
  http://www.joachim-breitner.de/<https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.joachim-breitner.de%2F&data=02%7C01%7Csimonpj%40microsoft.com%7Cbebbb53672104f8804a408d7770f544a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637108780029283726&sdata=CSR%2BQeNk7k5o6bjo%2Fv2Ke5Yu6h2hEwqR2gIcu0XMU5U%3D&reserved=0>

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