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<p class="MsoNormal">Friends<o:p></o:p></p>
<p class="MsoNormal">I’d like to move our record-syntax discussion forward. Link to
<a href="https://github.com/ghc-proposals/ghc-proposals/pull/282">proposal discussion</a>, and
<a href="https://mail.haskell.org/pipermail/ghc-steering-committee/2019-December/thread.html#1387">
December GHC steering committee debates</a>.<o:p></o:p></p>
<h2><b>No-space arguments<o:p></o:p></b></h2>
<p class="MsoNormal">I believe have agreed that <o:p></o:p></p>
<p class="MsoNormal"> f r.x<o:p></o:p></p>
<p class="MsoNormal">(with no spaces around the dot, and no parens around r.x) means<o:p></o:p></p>
<p class="MsoNormal"> f (r.x)<o:p></o:p></p>
<p class="MsoNormal">That is, treat it consistently with qualified names. I asked everyone to express a view; Iavor, Eric, Arnaud, Joachim, and Richard all said it was at least acceptable; others expressed no view. So let’s take that as a decision, at least
for now.<o:p></o:p></p>
<h2><b>Naked selectors<o:p></o:p></b></h2>
<p class="MsoNormal">Next question: how should we treat a “naked selector”, namely .x where there is no space after the dot, but there is a space before. I think there are three viable choices:<o:p></o:p></p>
<ol style="margin-top:0cm" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2"><b>It’s simply illegal</b>. This defers the choice; perhaps later we will have more experience to go on.<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2"><b>It’s a postfix operator</b>, binding less tightly than function application, but more tightly than any infix operator. So then (r .x) means r.x, and (r .x .y) means r.x.y. But
(f r .x) means (f r).x.<o:p></o:p></li></ol>
<p class="MsoListParagraph">This choice naturally supports chaining (nice to have, but not essential). We can write<o:p></o:p></p>
<p class="Code" style="margin-left:72.0pt">f .map double<o:p></o:p></p>
<p class="Code" style="margin-left:72.0pt"> .filter isEven<o:p></o:p></p>
<p class="MsoListParagraph">meaning <b><span style="font-family:"Courier New"">(f.map double).filter isEven</span></b><o:p></o:p></p>
<ol style="margin-top:0cm" start="3" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2"><b>It’s a postfix operator</b>, binding more tightly than function application, just as record update does. So then (f r .x) means (f r.x), and (f r .x .y s .z) means (f r.x.y s.z).<o:p></o:p></li></ol>
<p class="MsoListParagraph">This choice allows us to regard our decision about (f r.x) as what naturally happens if we parse it as three lexemes: f, r, and .x. But it also breaks the “function application binds more tightly than anything else” rule, just as
(f r {x=3}) sadly does already.<o:p></o:p></p>
<p class="MsoListParagraph">It does not permit chaining, at least not without stacked-up parens.<o:p></o:p></p>
<p class="MsoNormal">In all three cases we allow (.x), meaning (\r. r.x). For (2) and (3) we can regard it as a “section”, like infix operators only simpler because there is no argument.<o:p></o:p></p>
<p class="MsoNormal">I think this is the last major question we have to answer.<o:p></o:p></p>
<p class="MsoNormal">What are your views? Personally I lean towards (2), but I could certainly live with (1). I’m a bit reluctant to adopt (3).<o:p></o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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