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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Devs,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Shayan is working away on “Trees that grow”… do keep it on your radar:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">To:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ghc-devs
<br>
<b>Sent:</b> 25 May 2017 23:49<br>
<br>
</span><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;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:"Calibri",sans-serif;mso-fareast-language:EN-US">Do take a look at this:<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="font-size:11.0pt;font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">We propose to re-engineer HsSyn itself. This will touch a
<b>lot</b> of code.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="font-size:11.0pt;font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">But it’s very neat, and will bring big long-term advantages<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="font-size:11.0pt;font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">And we can do it a bit at a time<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;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:"Calibri",sans-serif;mso-fareast-language:EN-US">The wiki page
</span><a href="https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow">https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow</a>
<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">
has the details.</span> <a name="_MailEndCompose">I</a><span style="mso-bookmark:_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">t’s entirely an internal change, not a change to GHC’s specification,
so it’s independent of the GHC proposals process. But I’d value the opinion of other GHC devs</span></span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Meanwhile I have a question. When pretty-printing HsSyn we often have a situation like this:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="Code"> data Pass = Parsed | Renamed | Typechecked<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="Code"> data HsExpr (p :: Pass) = HsVar (IdP p) | ....<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="Code"> type famliy IdP p where<o:p></o:p></p>
<p class="Code"> IdP Parsed = RdrName<o:p></o:p></p>
<p class="Code"> IdP Renamed = Name<o:p></o:p></p>
<p class="Code"> IdP Typechecked = Id<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="Code"> instance (Outputable (IdP p)) => Outputable (HsExpr p) where<o:p></o:p></p>
<p class="Code"> ppr (HsVar v) = ppr v<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">The (ppr v) requires (Outputable (IdP p)), hence the context.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Moreover, and more seriously, there are things we just can't pretty-print<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">right now. For example, HsExpr has this data constructor:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="Code"> data HsExpr p = ...<o:p></o:p></p>
<p class="Code"> | OpApp (LHsExpr p)<o:p></o:p></p>
<p class="Code"> (LHsExpr p)<o:p></o:p></p>
<p class="Code"> (PostRn p Fixity)<o:p></o:p></p>
<p class="Code"> (LHsExpr p)<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">To pretty-print the third argument, we'd need to add<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="Code"> instance (Outputable (IdP p),<o:p></o:p></p>
<p class="Code"> Outputable (PostRn p Fixity)) -- New<o:p></o:p></p>
<p class="Code"> => Outputable (HsExpr p) where<o:p></o:p></p>
<p class="Code"> ppr (HsVar v) = ppr v<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">and that gets onerous.
<i>So today we never print these annotations</i>, to avoid bloating the instance contexts, which can be painful. It bit me yesterday.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">We have bitten that bullet for the Data class: look at HsExtension.DataId, which abbreviates the long list of dictionaries:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="Code"> type DataId p =<o:p></o:p></p>
<p class="Code"> ( Data p<o:p></o:p></p>
<p class="Code"> , ForallX Data p<o:p></o:p></p>
<p class="Code"> , Data (NameOrRdrName (IdP p))<o:p></o:p></p>
<p class="Code"> , Data (IdP p)<o:p></o:p></p>
<p class="Code"> , Data (PostRn p (IdP p))<o:p></o:p></p>
<p class="Code"> , Data (PostRn p (Located Name))<o:p></o:p></p>
<p class="Code"> , Data (PostRn p Bool)<o:p></o:p></p>
<p class="Code"> , Data (PostRn p Fixity)<o:p></o:p></p>
<p class="Code"> ,..and nine more... )<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Let me note in passing that [<a href="https://ghc.haskell.org/trac/ghc/wiki/QuantifiedContexts">wiki:QuantifiedContexts</a>] would make this somewhat shorter<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="Code"> type DataId p =<o:p></o:p></p>
<p class="Code"> ( Data p<o:p></o:p></p>
<p class="Code"> , ForallX Data p<o:p></o:p></p>
<p class="Code"> , Data (NameOrRdrName (IdP p))<o:p></o:p></p>
<p class="Code"> , Data (IdP p)<o:p></o:p></p>
<p class="Code"> , forall t. Data t => Data (PostRn p t))<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">But we still need one item in this list for each type function,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">and I am worried about how this scales to the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">[<a href="https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow">wiki:ImplementingTreesThatGrow</a>] story, when we have a type<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">function for each data constructor -- and there are a
<b>lot</b> of data<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">constructors.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-family:"Calibri",sans-serif">So I have four questions<o:p></o:p></span></b></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<ol style="margin-top:0cm" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo4"><span style="font-family:"Calibri",sans-serif">I think we should probably use a superclass instead of a type synonym<o:p></o:p></span></li></ol>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="Code">class (Data p, ForallX Data p, ....) => DataId p where {}<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoListParagraph"><span style="font-family:"Calibri",sans-serif">Why? Only one argument to pass, and to pass on to successive calls. I see no downside.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<ol style="margin-top:0cm" start="2" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo4"><span style="font-family:"Calibri",sans-serif">Shall we treat Outputable like Data? (I.e. make an abbreviation for a long list of Outputable instances and use it everywhere)<br>
<br>
<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo4"><span style="font-family:"Calibri",sans-serif">I thought of another way to do it (pass a token); see below<o:p></o:p></span></li></ol>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<ol style="margin-top:0cm" start="4" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo4"><span style="font-family:"Calibri",sans-serif">Are there any other ways?<o:p></o:p></span></li></ol>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-family:"Calibri",sans-serif">Token passing idea.<o:p></o:p></span></b></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Perhaps instead of passing lots of functions, we pass a singleton token<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">that encodes the pass, like this:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="Code"> instance (PassC p) => Outputable (HsExpr p) where<o:p></o:p></p>
<p class="Code"> ppr (HsVar v) = case getPass :: IsPass p of<o:p></o:p></p>
<p class="Code"> IsParsed -> ppr v<o:p></o:p></p>
<p class="Code"> IsRenamed -> ppr v<o:p></o:p></p>
<p class="Code"> IsTypechecked -> ppr v<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">The three ppr's are at different types, of course; that's the point.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">The infrastructure is something like<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="Code"> class PassC p where<o:p></o:p></p>
<p class="Code"> getPass :: IsPass p<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="Code"> data IsPass p where<o:p></o:p></p>
<p class="Code"> IsParsed :: IsPass Parsed<o:p></o:p></p>
<p class="Code"> IsRenamed :: IsParsed Renamed<o:p></o:p></p>
<p class="Code"> IsTypechecked :: IsParsed Typechecked<o:p></o:p></p>
<p class="Code"><o:p> </o:p></p>
<p class="Code"> instance PassC Parsed where getPass = IsParsed<o:p></o:p></p>
<p class="Code"> ...etc...<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Now we could sweep away all those OutputableX classes,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">replacing them with dynamic tests on the singletons IsParsed etc.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">This would have advantages:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">- Probably faster: there's a dynamic test, but many fewer dictionary<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"> arguments and higher-order function dispatch<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">- Only one dictionary to pass; programming is easier.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">The big downside is that it's not extensible: it works only because<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">we know the three cases. But the "Trees that Grow" story really doesn't<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">scale well to pretty-printing: so maybe we should just give up on that?<o:p></o:p></span></p>
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