<div dir="ltr">As far as I understand your explanation this should not lead to an error, although it is not the most obvious coercion. Is that right?<div><br></div><div>Do you or anybody else have a suggestion on how to resolve this issue?<br></div></div><div class="gmail_extra"><br><div class="gmail_quote">2015-11-18 17:13 GMT+00:00 Richard Eisenberg <span dir="ltr"><<a href="mailto:eir@cis.upenn.edu" target="_blank">eir@cis.upenn.edu</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div>Ah yes. I looked too quickly. Note that there are two NthCo's listed. Its the outermost that's the problem, which is deconstructing the Union. But it's doing so to prove that '["thres" :-> Int] ~ '["thres" :-> Int] which is rather easy to prove without NthCo. I'm not sure why GHC is doing this.</div><span class="HOEnZb"><font color="#888888"><div><br></div><div>Richard</div></font></span><div><div class="h5"><br><div><div>On Nov 18, 2015, at 12:11 PM, Jan Bracker <<a href="mailto:jan.bracker@googlemail.com" target="_blank">jan.bracker@googlemail.com</a>> wrote:</div><br><blockquote type="cite"><div dir="ltr">Hi Richard,<div><br></div><div>No "Split" is a class and is defined here: <a href="http://hackage.haskell.org/package/effect-monad-0.6.1/docs/Control-Effect-State.html#t:Split" target="_blank">http://hackage.haskell.org/package/effect-monad-0.6.1/docs/Control-Effect-State.html#t:Split</a></div><div>"Union" is a type function (synonym that refers to a type function call): <a href="http://hackage.haskell.org/package/effect-monad-0.6.1/docs/Control-Effect-Writer.html#t:Union" target="_blank">http://hackage.haskell.org/package/effect-monad-0.6.1/docs/Control-Effect-Writer.html#t:Union</a></div><div><br></div><div>thank you for your quick reply!</div><div><br></div><div>Best,</div><div>Jan</div></div><div class="gmail_extra"><br><div class="gmail_quote">2015-11-18 17:05 GMT+00:00 Richard Eisenberg <span dir="ltr"><<a href="mailto:eir@cis.upenn.edu" target="_blank">eir@cis.upenn.edu</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div>I took just a quick look at this. Is Split a type family? The NthCo coercion form takes apart a composite equality into its pieces. For example, if we know (Maybe a ~ Maybe b), then NthCo:0 will tell us that (a ~ b). In your case, it looks like GHC is trying to deduce (Union '["thres" :-> Int] []) ~ (Union '["thres" :-> Int] (Unit Reader)) from an equality of two (Split ...) types. If Split is a type family, this deduction is unsound. That may be what Core Lint is worried about.</div><div><br></div><div>I'm not surprised that the executable would run with an error. But it might not in the future. If -dcore-lint fails, it means that there is a potential type safety issue in the Core code, and this should be taken seriously.</div><div><br></div><div>I hope this helps!</div><div>Richard</div><br><div><div><div><div>On Nov 18, 2015, at 11:35 AM, Jan Bracker <<a href="mailto:jan.bracker@googlemail.com" target="_blank">jan.bracker@googlemail.com</a>> wrote:</div><br></div></div><blockquote type="cite"><div><div><div dir="ltr">Hi,<div><br></div><div>I am using the type checker plugin interface and I am trying to produce some evidence for type class instances. During compilation of one of my examples I get this core-lint error:</div><div><br></div><div><div>*** Core Lint errors : in result of Simplifier ***</div><div><no location info>: Warning:</div><div> [RHS of ds_a6bY :: (Set '["thres" :-> Int], Set (Unit Reader))]</div><div> Bad getNth:</div><div> Nth:0</div><div> (Nth:2</div><div> (Sub (Sym (TFCo:R:Inv[]Readerfg[0] <'["thres" :-> Int]>_N <'[]>_N))</div><div> ; (Inv</div><div> <Reader>_N <'["thres" :-> Int]>_N (Sym TFCo:R:Unit[]Reader[0]))_R</div><div> ; Sub</div><div> (TFCo:R:Inv[]Readerfg[0] <'["thres" :-> Int]>_N <Unit Reader>_N)))</div><div> Split '["thres" :-> Int] '[] (Union '["thres" :-> Int] '[])</div><div> Split</div><div> '["thres" :-> Int]</div><div> (Unit Reader)</div><div> (Union '["thres" :-> Int] (Unit Reader))</div></div><div><br></div><div>I suppose "getNth" refers to the constructor "EvTupleSel" from "EvTerm", "TcNthCo" from "TcCoercion" or to "NthCo" from "Coercion". But I never produce evidence of the shape "getNth". My evidence production code can be found at [1] and the only place where evidence of this shape can come from is my "evaluateType" function [2] that calls "normaliseType" from the GHC module "FamInstEnv". You can reproduce the error by checking out commit 144525886ec107af6f1283b26b19f8125c980aa4 from [3] and running "make effect-example" in the top directory of the repository (GHC 7.10 or better is required and a sandbox is automatically created).</div><div><br></div><div>The core-lint error does not seem to have any negative consequences when ignored. The produced executable works fine. Can somebody explain why it appears and maybe how I can fix it?</div><div><br></div><div>Thank you!</div><div>Jan</div><div><br></div><div>[1] <a href="https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b19f8125c980aa4/src/Control/Polymonad/Plugin/Evidence.hs#L177" target="_blank">https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b19f8125c980aa4/src/Control/Polymonad/Plugin/Evidence.hs#L177</a></div><div>[2] <a href="https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b19f8125c980aa4/src/Control/Polymonad/Plugin/Evaluate.hs#L29" target="_blank">https://github.com/jbracker/polymonad-plugin/blob/144525886ec107af6f1283b26b19f8125c980aa4/src/Control/Polymonad/Plugin/Evaluate.hs#L29</a></div><div>[3] <a href="https://github.com/jbracker/polymonad-plugin" target="_blank">https://github.com/jbracker/polymonad-plugin</a></div></div></div></div>
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