<div dir="auto">Don't pay *too* much attention to mkWeakIORef in particular; for some reason it only makes a weak reference from an IORef to itself. It *should* have been written<div dir="auto"><br></div><div dir="auto"><div dir="auto">mkWeakIORef :: IORef a -> b -> IO () -> IO (Weak (IORef a))</div><div dir="auto">mkWeakIORef r@(IORef (STRef r#)) b (IO finalizer) = IO $ \s -></div><div dir="auto"> case mkWeak# r# b finalizer s of (# s1, w #) -> (# s1, Weak w #)</div><div dir="auto"><br></div><div dir="auto">and the current version should've been</div><div dir="auto"><br></div><div dir="auto">mkSimpleWeakIORef r fin = mkWeakIORef r r fin</div></div></div><br><div class="gmail_quote"><div dir="ltr">On Thu, Aug 23, 2018, 5:52 PM David Feuer <<a href="mailto:david.feuer@gmail.com">david.feuer@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto"><div>Weak pointers to unlifted heap objects are the opposite of dodgy! We have base library functions (e.g., Data.IORef.mkWeakIORef) for creating weak references to lifted objects that *actually* create references to the underlying unlifted objects instead. This is much more reliable, because the wrapper (e.g., the STRef constructor) could be stripped away by worker-wrapper, while the underlying object (e.g., the MutVar#) won't be.</div><div dir="auto"><br><div class="gmail_quote" dir="auto"><div dir="ltr">On Thu, Aug 23, 2018, 5:43 PM Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank" rel="noreferrer">simonpj@microsoft.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span>Gah. We have no way to be polymorphic over all pointers (both lifted and unlifted) but not over Int# etc.<u></u><u></u></span></p>
<p class="MsoNormal"><span><u></u> <u></u></span></p>
<p class="MsoNormal"><span>As you say, this is too much of a special case to make an invasive change.<u></u><u></u></span></p>
<p class="MsoNormal"><span><u></u> <u></u></span></p>
<p class="MsoNormal"><span>I’m quite dubious about making weak poitners to unlifted heap-allocated objects. I can’t say it’s wrong but it feels dodgy to me.<u></u><u></u></span></p>
<p class="MsoNormal"><span><u></u> <u></u></span></p>
<p class="MsoNormal"><span>Simon<u></u><u></u></span></p>
<p class="MsoNormal"><span><u></u> <u></u></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> David Feuer <<a href="mailto:david.feuer@gmail.com" rel="noreferrer noreferrer" target="_blank">david.feuer@gmail.com</a>>
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<b>Sent:</b> 22 August 2018 14:59<br>
<b>To:</b> Ben Gamari <<a href="mailto:ben@smart-cactus.org" rel="noreferrer noreferrer" target="_blank">ben@smart-cactus.org</a>><br>
<b>Cc:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" rel="noreferrer noreferrer" target="_blank">simonpj@microsoft.com</a>>; David Feuer <<a href="mailto:david@well-typed.com" rel="noreferrer noreferrer" target="_blank">david@well-typed.com</a>>; ghc-devs <<a href="mailto:ghc-devs@haskell.org" rel="noreferrer noreferrer" target="_blank">ghc-devs@haskell.org</a>><br>
<b>Subject:</b> Re: Unlifted primop types<u></u><u></u></span></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">The problem is that this also accepts things that aren't pointers at all! We could fix that *for primops* by changing RuntimeRep to something like<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">data RuntimeRep<u></u><u></u></p>
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<p class="MsoNormal"> = PtrRep Liftedness<u></u><u></u></p>
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<p class="MsoNormal"> | ...<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">But that would only work for primops (at least for now) so it may not be worth the breakage.<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">On Wed, Aug 22, 2018, 7:45 AM Ben Gamari <<a href="mailto:ben@smart-cactus.org" rel="noreferrer noreferrer" target="_blank">ben@smart-cactus.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" rel="noreferrer noreferrer" target="_blank">simonpj@microsoft.com</a>> writes:<br>
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> | Huh! It looks like what we currently do for some primops is just use a<br>
> | totally bogus kind. For example, mkWeak# will happily accept an Int# as<br>
> | its first argument.<br>
><br>
> Well, I see<br>
> primop MkWeakOp "mkWeak#" GenPrimOp<br>
> o -> b -> (State# RealWorld -> (# State# RealWorld, c #))<br>
><br>
> and I believe (from Ben's message) that the "o" means "open type variable",<br>
> which is the old terminology for what we now call levity-polymorphic.<br>
><br>
Right; currently (largely for historical reasons) we use `o` to<br>
accommodate cases that accept both lifted and unlifted pointers. <br>
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Cheers,<br>
<br>
- Ben<u></u><u></u></p>
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