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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">I’m mildly against. I see the motivation but I’m doubtful that this is the way to achieve it. See my comment on the ticket.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ghc-steering-committee [mailto:ghc-steering-committee-bounces@haskell.org]
<b>On Behalf Of </b>Simon Marlow<br>
<b>Sent:</b> 08 May 2017 09:52<br>
<b>To:</b> ghc-steering-committee@haskell.org<br>
<b>Subject:</b> [ghc-steering-committee] Proposal #46: NOUNPACK pragmas for function arguments<o:p></o:p></span></p>
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<a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F46&data=02%7C01%7Csimonpj%40microsoft.com%7C09b74fabee624f440e5308d495efb547%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636298304152679025&sdata=SbVC4Kd2xTK%2F%2BNtlC5w5JBTWKJSKzTD79Kttde0OjM4%3D&reserved=0">https://github.com/ghc-proposals/ghc-proposals/pull/46</a><o:p></o:p></p>
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I tentatively propose that we accept this proposal.<o:p></o:p></p>
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Briefly, the proposal is to allow a NOUNPACK pragma in a *type signature* to indicate that a value should not be unpacked by the worker-wrapper transformation, e.g.:<o:p></o:p></p>
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<o:p> </o:p></p>
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insertR :: Ord k => {-# NOUNPACK #-} k -> v -> Map k v -> Map k v<o:p></o:p></p>
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insertR k v m = go k k v m<o:p></o:p></p>
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where<o:p></o:p></p>
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go :: Ord k => {-# NOUNPACK #-} k -> k -> v -> Map k v -> Map k v<o:p></o:p></p>
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go kp k v m = ...<o:p></o:p></p>
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<o:p> </o:p></p>
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The idea is that sometimes unpacking a strict argument leads to worse code because the repacking doesn't get simplified away, and it's hard for worker-wrapper to tell that up front. It's quite difficult for the programmer to avoid this problem currently - the
one trick we have available is the `lazy` pseudo-function, but that's difficult to use for this purpose because you have to ensure that *all* uses of the argument are made lazy, and there can be collateral damage. It's the wrong tool to solve this problem,
but it's the only tool we have.<o:p></o:p></p>
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<o:p> </o:p></p>
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I've had this issue personally, and I've worked around it with `lazy`, but it's an ugly and brittle workaround, and something better is needed.<o:p></o:p></p>
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<o:p> </o:p></p>
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If we assume that NOUNPACK can only occur on the left of an arrow (rather like UNPACK/NOUNPACK in a GADT signature), then it's a fairly straightforward extension, and just requires plumbing the information through to the demand analyser / worker-wrapper.<o:p></o:p></p>
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<o:p> </o:p></p>
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The dual UNPACK annotation also makes sense, I think, although it's not part of this proposal. I can imagine it being a future extension.<o:p></o:p></p>
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<o:p> </o:p></p>
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Drawbacks I see:<o:p></o:p></p>
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- It doesn't let you say anything more detailed than just "no", e.g. you can't say "don't unpack the first field of the record"<o:p></o:p></p>
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- It requires a type signature, which you wouldn't otherwise need. I suppose you could use PartialTypeSignatures.<o:p></o:p></p>
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- It doesn't let you say anything about strictness, only unpacking. If a NOUNPACK argument is inferred by the demand analyser to be strict, callers can use call-by-value.<o:p></o:p></p>
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<o:p> </o:p></p>
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Overall I don't see any fatal flaws and there are benefits. It's not a beautiful extension by any means, but I don't think it's expensive or complicated to implement. I suggest we accept it. Thoughts?<o:p></o:p></p>
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<o:p> </o:p></p>
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Cheers<o:p></o:p></p>
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Simon<o:p></o:p></p>
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