<div dir="ltr"><div>Arrows haven't seen much love for a while. In part this is because many of the original applications for arrows have been shown to be perfectly suited to being handled by Applicatives. e.g. the Swiestra/Duponcheel parser that sort of kickstarted everything.</div><div><br></div>There are several options for improved arrow desugaring.<div><br></div><div>Megacz's work on GArrows at first feels like it should be applicable here, as it lets you change out the choice of pseudo-product while preserving the general arrow feel. Unfortunately, the GArrow class isn't sufficient for most arrow desguaring, due to the fact that the arrow desugaring inherently involves breaking apart patterns for almost any non-trivial use and nothing really requires the GArrow 'product' to actually even be product like.</div><div><br></div><div><div><div>Cale Gibbard and Ryan Trinkle on the other hand like to use a more CCC-like basis for arrows. This stays in the spirit to the GArrow class, but you still have the problems around pattern matching. I don't think they actually wrote anything to deal with the actual arrow notation and just programmed in the alternate style to get better introspection on the operations involved. I think the key insight there is that much of the notation can be made to work with weaker categorical structures than full arrows, but the existing class hierarchy around arrows is very coarse.</div><div><br>As a minor data point both of these sorts of encodings of arrow problems start to drag in language extensions that make the notation harder to standardize. Currently they work with bog standard Haskell 98/2010.<br><br></div></div><div>If you're looking for an interesting theoretical direction to extend Arrow notation:</div><div><br></div><div>An arrow is a strong monad in the category of profunctors [1]. </div><div><br></div><div>Using the profunctors library [2] (Strong p, Category p) is equivalent in power to Arrow p.</div><div><br></div><div>Exploiting that, a profunctor-based desugaring could get away with much weaker constraints than Arrow depending on how much of proc notation you use.<br></div></div><div><br></div><div>Alternately a separate class hierarchy that only required covariance in the second argument is an option, but my vague recollection from the last time that I looked into this is that while such a desguaring only uses covariance in the second argument of the profunctor, you can prove that contravariance in the first argument follows from the pile of laws. This subject came up the last time someone thought to extend the Arrow desguaring. You can probably find a thread on the mailing list from Ross Paterson a few years ago.</div><div><br></div><div>This version has the benefit of fitting pretty close to the existing arrow desugaring and not needing new language extensions.</div><div><br></div><div>On the other hand, refactoring the Arrow class in this (or any other) way is somewhat of an invasive exercise. The profunctors package offers moral equivalents to most of the Arrow subclasses, but no effort has been made to match the existing Arrow hierarchy.</div><div><br></div><div>Given that little new code seems to be being written with Arrows in mind, while some older code makes heavy use of it (hxt, etc.), refactoring the arrow hierarchy is kind of a hard sell. It is by no means impossible, just something that would require a fair bit of community wrangling and a lot of work showing clear advantages to a new status quo at a time when its very hard to get anybody to care about arrow notation at all.</div><div><br></div><div>-Edward</div><div><br></div><div>[1] <a href="http://www-kb.is.s.u-tokyo.ac.jp/~asada/papers/arrStrMnd.pdf">http://www-kb.is.s.u-tokyo.ac.jp/~asada/papers/arrStrMnd.pdf</a><br></div><div>[2] <a href="http://hackage.haskell.org/package/profunctors-5.2/docs/Data-Profunctor-Strong.html">http://hackage.haskell.org/package/profunctors-5.2/docs/Data-Profunctor-Strong.html</a></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Dec 2, 2016 at 10:57 AM, Jan Bracker via ghc-devs <span dir="ltr"><<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Simon, Richard,<div><br></div><div>thank you for your answer! I don't have time to look into the GHC sources right now, but I will set aside some time after the holidays and take a close look at what the exact restrictions on proc-notation are and document them.</div><div><br></div><div>Since you suggested a rewrite of GHC's handling of proc-syntax, are there any opinions on integrating generalized arrows (Joseph 2014) in the process? I think they would greatly improve arrows! I don't know if I have the time to attempt this, but if I find the time I would give it a try. Why wasn't this integrated while it was still actively developed?</div><div><br></div><div>Best,</div><div>Jan</div><div><br></div><div>[Joseph 2014] <a href="https://www2.eecs.berkeley.edu/Pubs/TechRpts/2014/EECS-2014-130.pdf" target="_blank">https://www2.eecs.berkel<wbr>ey.edu/Pubs/TechRpts/2014/<wbr>EECS-2014-130.pdf</a><br><div><br></div><div><br></div></div></div><div class="HOEnZb"><div class="h5"><div class="gmail_extra"><br><div class="gmail_quote">2016-11-29 12:41 GMT+00:00 Simon Peyton Jones <span dir="ltr"><<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div lang="EN-GB" link="blue" vlink="purple">
<div class="m_34741211521753758m_6725992496896091287WordSection1">
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Jan,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Type checking and desugaring for arrow syntax has received Absolutely No Love for several years. I do not understand how it works very well, and I would not be at all surprised if it is broken
in corner cases.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">It really needs someone to look at it carefully, document it better, and perhaps refactor it – esp by using a different data type rather than piggy-backing on HsExpr.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">In the light of that understanding, I think rebindable syntax will be easier.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">I don’t know if you are up for that, but it’s a rather un-tended part of GHC.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Thanks<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Simon<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><u></u> <u></u></span></p>
<div style="border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt">
<div>
<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 [mailto:<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">ghc-devs-bounces@haske<wbr>ll.org</a>]
<b>On Behalf Of </b>Richard Eisenberg<br>
<b>Sent:</b> 28 November 2016 22:30<br>
<b>To:</b> Jan Bracker <<a href="mailto:jan.bracker@googlemail.com" target="_blank">jan.bracker@googlemail.com</a>><br>
<b>Cc:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Help needed: Restrictions of proc-notation with RebindableSyntax<u></u><u></u></span></p>
</div>
</div><div><div class="m_34741211521753758h5">
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<p class="MsoNormal">Jan’s question is a good one, but I don’t know enough about procs to be able to answer. I do know that the answer can be found by looking for uses of `tcSyntaxOp` in the TcArrows module.... but I just can’t translate it all to source Haskell,
having roughly 0 understanding of this end of the language.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">Can anyone else help Jan here?<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">Richard<u></u><u></u></p>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal">On Nov 23, 2016, at 4:34 AM, Jan Bracker via ghc-devs <<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>> wrote:<u></u><u></u></p>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<div>
<p class="MsoNormal">Hello,<u></u><u></u></p>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">I want to use the proc-notation together with RebindableSyntax. So far what I am trying to do is working fine, but I would like to know what the exact restrictions on the supplied functions are. I am introducing additional indices and constraints
on the operations. The documentation [1] says the details are in flux and that I should ask directly.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">Best,<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">Jan<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">[1] <a href="https://na01.safelinks.protection.outlook.com/?url=https:%2F%2Fdownloads.haskell.org%2F~ghc%2Flatest%2Fdocs%2Fhtml%2Fusers_guide%2Fglasgow_exts.html%23rebindable-syntax-and-the-implicit-prelude-import&data=02%7C01%7Csimonpj%40microsoft.com%7C80e472cedf78463bd18408d417de1af5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636159690104764444&sdata=ygqePSmgcPKnPmKDBfZplkyjG9BIDBO1L1MWHNpqw88%3D&reserved=0" target="_blank">https://downloads.haskell.<wbr>org/~ghc/latest/docs/html/user<wbr>s_guide/glasgow_exts.html#rebi<wbr>ndable-syntax-and-the-implicit<wbr>-prelude-import</a><u></u><u></u></p>
</div>
</div>
<p class="MsoNormal">______________________________<wbr>_________________<br>
ghc-devs mailing list<br>
<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs" target="_blank">http://mail.haskell.org/cgi-bi<wbr>n/mailman/listinfo/ghc-devs</a><u></u><u></u></p>
</div>
</blockquote>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
</div></div></div>
</div>
</div>
</blockquote></div><br></div>
</div></div><br>______________________________<wbr>_________________<br>
ghc-devs mailing list<br>
<a href="mailto:ghc-devs@haskell.org">ghc-devs@haskell.org</a><br>
<a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs" rel="noreferrer" target="_blank">http://mail.haskell.org/cgi-<wbr>bin/mailman/listinfo/ghc-devs</a><br>
<br></blockquote></div><br></div>