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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Try <a href="https://www.microsoft.com/en-us/research/publication/levity-polymorphism/">
this paper</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">The function type constructor actually has type<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"> (->) :: forall (r1::RuntimeRep) (r2::RuntimeRep). TYPE r1 -> TYPE r2 -> TYPE LiftedRep<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">But when it appears applied to two argument, thus ((->) r1 r2 t1 t2), we print it infix (t1 -> t2).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<span style="mso-fareast-language:EN-US">So </span><span style="font-family:"Courier New""> (-> 'GHC.Types.LiftedRep 'GHC.Types.LiftedRep thing thing)</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt;mso-fareast-language:EN-US">is the same as (thing -> thing).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;mso-fareast-language:EN-US">I cannot explain why it is being printed in that non-infix way, but your email doesn’t give enough context to know.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;mso-fareast-language:EN-US">I hope this helps a bit. Where did you look for information? I’d like to add pointers that would have been helpful to you.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Ranjit Jhala <jhala@cs.ucsd.edu>
<br>
<b>Sent:</b> 24 April 2019 04:56<br>
<b>To:</b> ghc-devs@haskell.org; Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Subject:</b> Puzzled by GHC.Types.LiftedRep<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<span style="font-family:"Courier New"">Hi all,<o:p></o:p></span></p>
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<span style="font-family:"Courier New""><o:p> </o:p></span></p>
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<span style="font-family:"Courier New"">Consider the following program:<o:p></o:p></span></p>
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<span style="font-family:"Courier New""><o:p> </o:p></span></p>
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<span style="font-family:"Courier New"">```<o:p></o:p></span></p>
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<span style="font-family:"Courier New"">import Control.Arrow (second)</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">import qualified Control.Monad.Trans.Reader as R</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">myLocal :: (r -> r) -> R.ReaderT r m a -> R.ReaderT r m a</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">myLocal = undefined</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">foo :: (thing -> thing) -> R.ReaderT (d, thing) m a -> R.ReaderT (d, thing) m a</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">foo arg = myLocal (second arg)</span><o:p></o:p></p>
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<span style="font-family:"Courier New"">```<br>
<br>
When compiled with GHC 8.6.4, I get the odd behavior that </span><o:p></o:p></p>
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<span style="font-family:"Courier New"">at the call `second arg`, the type of `arg` is just:</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New""> thing -> thing </span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">but the *input* type of `second` (after applying the <o:p></o:p></span></p>
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<span style="font-family:"Courier New"">various type and dictionary parameters) is something <o:p></o:p></span></p>
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<span style="font-family:"Courier New"">that looks like<o:p></o:p></span></p>
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<span style="font-family:"Courier New""><o:p> </o:p></span></p>
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<span style="font-family:"Courier New"">while the *input* type of `second` (after the various type and dictionary applications etc.) is the mysterious (to me):</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New""> (-> 'GHC.Types.LiftedRep 'GHC.Types.LiftedRep thing thing)</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">Can someone explain or point me to some documentation or </span><o:p></o:p></p>
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<span style="font-family:"Courier New"">paper that describes what the above means? Specifically, </span><o:p></o:p></p>
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<span style="font-family:"Courier New"">how/why does GHC "equate" the two things above in order </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Courier New"">to consider the core well-formed?</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">Many thanks in advance!</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">- Ranjit.</span><o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New""><o:p> </o:p></span></p>
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<span style="font-family:"Courier New""><o:p> </o:p></span></p>
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