<div dir="ltr"><div>I recommend that we <i><b>accept</b></i> proposal #265 (Unlifted Datatypes)<br></div><div><br></div><div><a href="https://github.com/ghc-proposals/ghc-proposals/pull/265">https://github.com/ghc-proposals/ghc-proposals/pull/265</a></div><div></div><div><a href="https://github.com/sgraf812/ghc-proposals/blob/unlifted-data/proposals/0000-unlifted-datatypes.rst" rel="noreferrer" target="_blank">https://github.com/sgraf812/ghc-proposals/blob/unlifted-data/proposals/0000-unlifted-datatypes.rst</a></div><div><br></div><div>It's a fairly conservative extension: the kind <span style="font-family:monospace">TYPE 'UnliftedRep</span> 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.</div><div><br></div><div>There are considerable performance benefits to be had for performance-critical code, for instance the containers package.<br></div><div><br></div><div>A couple of minor issues remain:</div><ul><li>Without special support, the type <span style="font-family:monospace">data unlifted Strict a = Force !a</span> comes with an associated box, so this type isn't as useful as it could be.</li><li>It isn't possible to define values of kind <span style="font-family:monospace">TYPE 'UnlifedRep</span> 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 <span style="font-family:monospace">TYPE 'UnlifedRep</span> to the top level in the optimiser, which can lead to surprising performance behaviour. See <a href="https://gitlab.haskell.org/ghc/ghc/issues/17521">https://gitlab.haskell.org/ghc/ghc/issues/17521</a></li></ul><div>Nevertheless, we shouldn't let the perfect be the enemy of the good, and Unlifted Datatypes is a clearly useful addition in my view.</div><div><br></div><div>Cheers</div><div>Simon<br></div><div><br></div><div><br></div><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, 28 Nov 2019 at 10:06, Joachim Breitner <<a href="mailto:mail@joachim-breitner.de">mail@joachim-breitner.de</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Dear Committee,<br>
<br>
this is your secretary speaking:<br>
<br>
Unlifed Datatypes<br>
has been proposed by Sebastian Graf<br>
<a href="https://github.com/ghc-proposals/ghc-proposals/pull/265" rel="noreferrer" target="_blank">https://github.com/ghc-proposals/ghc-proposals/pull/265</a><br>
<a href="https://github.com/sgraf812/ghc-proposals/blob/unlifted-data/proposals/0000-unlifted-datatypes.rst" rel="noreferrer" target="_blank">https://github.com/sgraf812/ghc-proposals/blob/unlifted-data/proposals/0000-unlifted-datatypes.rst</a><br>
<br>
I propose Simon Marlow as the shepherd, as the expert on low-level stuff.<br>
<br>
Please reach consensus as described in<br>
<a href="https://github.com/ghc-proposals/ghc-proposals#committee-process" rel="noreferrer" target="_blank">https://github.com/ghc-proposals/ghc-proposals#committee-process</a><br>
I suggest you make a recommendation, in a new e-mail thread with the<br>
proposal number in the subject, about the decision, maybe point out<br>
debatable points, and assume that anyone who stays quiet agrees with<br>
you.<br>
<br>
Thanks,<br>
Joachim<br>
-- <br>
Joachim Breitner<br>
  <a href="mailto:mail@joachim-breitner.de" target="_blank">mail@joachim-breitner.de</a><br>
  <a href="http://www.joachim-breitner.de/" rel="noreferrer" target="_blank">http://www.joachim-breitner.de/</a><br>
<br>
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</blockquote></div></div>