I recommend that we *accept* proposal #265 (Unlifted Datatypes)
https://github.com/ghc-proposals/ghc-proposals/pull/265
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
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
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/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 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 http://www.joachim-breitner.de/
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