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

I’m generally in support of this idea, but I’ve suggested a few tweaks on the GitHub thread.

On Dec 2, 2019, at 10:06 AM, Simon Marlow wrote:

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

https://github.com/ghc-proposals/ghc-proposals/pull/265 https://github.com/ghc-proposals/ghc-proposals/pull/265 https://github.com/sgraf812/ghc-proposals/blob/unlifted-data/proposals/0000-... https://github.com/sgraf812/ghc-proposals/blob/unlifted-data/proposals/0000-...

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://gitlab.haskell.org/ghc/ghc/issues/17521 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 mailto:mail@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://github.com/ghc-proposals/ghc-proposals/pull/265 https://github.com/sgraf812/ghc-proposals/blob/unlifted-data/proposals/0000-... https://github.com/sgraf812/ghc-proposals/blob/unlifted-data/proposals/0000-...

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://github.com/ghc-proposals/ghc-proposals#committee-process 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@joachim-breitner.de mailto:mail@joachim-breitner.de http://www.joachim-breitner.de/ http://www.joachim-breitner.de/

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