<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>It seems like we may be on the same page now. Do you agree with my previous email that the change to the GND check will not solve *all* your problems? That is, there will be some derived instances you will have to hand-write, but (I think) for good reason.</div><div><br></div><div>Richard</div><br><div><div>On Oct 16, 2013, at 12:46 PM, Edward Kmett wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><meta http-equiv="content-type" content="text/html; charset=utf-8"><div dir="auto"><div><br></div><div><br>On Oct 16, 2013, at 9:28 AM, Simon Peyton-Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>> wrote:<br><br></div><blockquote type="cite"><div>
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<div class="WordSection1"><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">I think I know what to do about the GND + roles question.<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">First, the problem</span></b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">.
Here’s an example:<br>
</span><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> class Trans t a where<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> foo :: t a -> t a<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> newtype N x = MkN x deriving( Trans Maybe )<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">As things stand,
</span><b><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US">Trans</span></b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> gets roles (representational,
nominal). The second parameter ‘a’ gets a nominal role because we don’t know what ‘t’ will get instantiated too.<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">As a result the attempted GND is rejected because of the new role stuff. But with 7.6.3 we’ll get an instance
<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> instance Trans Maybe a => Trans Maybe (N a)<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">from the GND mechanism.</span><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"><o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Do I have this right?
<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Second, there is a Good Reason for the problem</span></b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">.
Suppose I said<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> newtype N x = MkN x deriving( Trans D )<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">where D was a data family. Then the foo method for (D x) might seg-fault if applied to a (D (N x)) value. So the current GND is
treading on very thin ice, and it is ice that the author (Edward) does not control. His clients control it.<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Do I have this right?<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p></div></div></blockquote><blockquote type="cite"><div class="WordSection1"><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Third, there is an easy solution</span></b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">.
As Richard observed in a separate thread about Coercible, GND does not attempt to build a coercion to witness<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> Trans Maybe x ~R Trans Maybe (N x)<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Rather, it builds a dictionary for (Trans Maybe (N x)) thus<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> dTransMaybeN :: Trans Maybe x -> Trans Maybe (N x)<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> dTransMaybeN d = MkTrans (sel_foo d |> co)<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">where the (sel_foo d) selects the foo method from d. That is, we actually cast the methods, not the dictionary as a whole. So
what we need is a coercion between<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> Maybe x ~R Maybe (N x)<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Can we get that? Yes of course! Maybe has a representational role.<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Bottom line: we should accept or reject a GND attempt not on the basis of the role of the class, but rather on whether the method
types are coercible. </span></b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Indeed, this is just what we agreed some days ago<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><a href="http://www.haskell.org/pipermail/glasgow-haskell-users/2013-October/024386.html">http://www.haskell.org/pipermail/glasgow-haskell-users/2013-October/024386.html</a><o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">So I think this solves Edward’s problem below.<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Moreover, I think this solves the other failures in
<a href="http://www.haskell.org/pipermail/ghc-devs/2013-October/002961.html">http://www.haskell.org/pipermail/ghc-devs/2013-October/002961.html</a>. Here is one example, in that email. smallcheck has this:<o:p></o:p></span></p><p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US">newtype Series m a = Series (ReaderT Depth (LogicT m) a)<o:p></o:p></span></p><p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> deriving<o:p></o:p></span></p><p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> ( …, MonadLogic)<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">where logict defines MonadLogic thus:<o:p></o:p></span></p><p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US">class (MonadPlus m) => MonadLogic m where<o:p></o:p></span></p><p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> msplit :: m a -> m (Maybe (a, m a))<o:p></o:p></span></p><p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">So the “bottom line” check above will attempt to find a cocercion betwem msplit’s type with m=Series m, and with m=ReaderT Depth
(LogitT m). Right? So on the left of msplit’s arrow, we’ll ask can we prove<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D;mso-fareast-language:EN-US"> Series m a ~R ReaderT Depth (LogicT m) a<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Can we show that? Yes, of course… that is the very newtype coercion for Series.<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">In short, I think we are fine, once Richard has implemented the new GND test.<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Am I wrong?</span></p></div></blockquote><br><div><div>I have no counter examples for GND based on coercing dictionaries rather than parameters at this time and I do agree that this does strike me as sufficient to plug the obvious holes in the current approach.</div><div><br></div><div>I'll keep looking and see if I can't find a way to subvert the system, but this sounds promising for roles. </div><div><br></div><div>In light of this, the NPR approach that Richard suggested strikes me as more relevant to fixing up Coercible than GND and as you mentioned we can very well spend another release cycle to get Coercible right.<br><br>Assuming the dictionary casts work the way we expect, I'm think I'm sold. The main thing I don't want to do is wake up after 7.8 is cut and personally have to write 4000 lines of previously derived instances that are provably safe, because of an overly restrictive role scheme. I'm cautiously optimistic that this will be sufficient to keep me out of that scenario. ;)<br><br>-Edward</div></div><br><blockquote type="cite"><div><div class="WordSection1"><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm"><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-devs [<a href="mailto:ghc-devs-bounces@haskell.org">mailto:ghc-devs-bounces@haskell.org</a>]
<b>On Behalf Of </b>Edward Kmett<br>
<b>Sent:</b> 13 October 2013 23:02<br>
<b>To:</b> ghc-devs; Richard Eisenberg<br>
<b>Subject:</b> Re: More GND + role inference woes<o:p></o:p></span></p>
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I didn't think I was using GND much at all, but apparently I was wrong.<o:p></o:p></p>
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After Ben's initial foray into building linear, I went and looked at the other applications of GeneralizedNewtypeDeriving in my code and found that in parsers, all of the derived instances for the supplied parser transformers fail.<o:p></o:p></p>
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This also affects any attempt to use GND to do deriving for any monad transformer stack that isn't fully instantiated to concrete terms. This is actually a very common idiom to avoid writing boilerplate on top of transformers:<o:p></o:p></p>
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<o:p> </o:p></p>
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newtype T m a = T { runT : StateT MyState (ReaderT MyEnv) m a }<o:p></o:p></p>
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deriving (Functor, Monad, Applicative, MonadState MyState, MonadReader MyEnv, ...)<o:p></o:p></p>
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<o:p> </o:p></p>
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As I rummage through more of my code, I actually can't find any instances of GND that do still work that aren't of the form:<o:p></o:p></p>
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<o:p> </o:p></p>
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newtype Foo a = Foo { runFoo :: a } deriving ...<o:p></o:p></p>
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<o:p> </o:p></p>
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<div><p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Literally every other example I have of GND in the code I maintain has something in it that causes it to run afoul of the new roles machinery.<o:p></o:p></p>
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<o:p> </o:p></p>
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I'd say the problem is more widespread than we thought.<o:p></o:p></p>
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<o:p> </o:p></p>
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<div><p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
-Edward<o:p></o:p></p>
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<o:p> </o:p></p>
<div><p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
On Sun, Oct 13, 2013 at 5:26 PM, Edward Kmett <<a href="mailto:ekmett@gmail.com" target="_blank">ekmett@gmail.com</a>> wrote:<o:p></o:p></p>
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<div><p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Ben Gamari was trying to update my linear package to work with GHC HEAD.<o:p></o:p></p>
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<o:p> </o:p></p>
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<div><p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Along the way he noted an example of the new GND+role inference machinery failing rather spectacularly.<o:p></o:p></p>
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<o:p> </o:p></p>
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<a href="http://hackage.haskell.org/package/linear-1.3/docs/src/Linear-Affine.html#Point" target="_blank">http://hackage.haskell.org/package/linear-1.3/docs/src/Linear-Affine.html#Point</a><o:p></o:p></p>
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<o:p> </o:p></p>
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Note the number of classes that the current proposal would force us to hand implement. <o:p></o:p></p>
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<o:p> </o:p></p>
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<div><p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
=(<o:p></o:p></p>
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<div><p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="color:#888888"><o:p> </o:p></span></p>
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<div><p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="color:#888888">-Edward<o:p></o:p></span></p>
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</blockquote>
</div><p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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