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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:36.0pt">
Where specifying the type instance with a wild card keeps GHC happy (the XXOverLit case), but specifying for each of the three constructors for pass does not (the XEOverLit case)<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Well, of course! The derived data instance looks something like<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt">instance (Data (GhcPass p)) => Data (Experiment (GhcPass p)) where<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:36.0pt"> gfoldl = ….Needs (Data (XEOverLit (GhcPass p)))…<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">How can GHC solve the wanted constraint<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> Data (XEOverlit (GhcPass p))<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">With three equations, it can’t. With one, it can.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">So why not use one? <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><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"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Alan & Kim Zimmerman<br>
<b>Sent:</b> 13 November 2017 09:24<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Trees that Grow and constraints<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:12.0pt;margin-left:0cm">
At the moment the Trees that Grow implementation in GHC master makes use of massive constraint types to provide Data instances for the hsSyn AST.<o:p></o:p></p>
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I am trying to remove the need for this, and have hit a problem.<o:p></o:p></p>
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The example I have reduced it to is here [1]<o:p></o:p></p>
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The essence of the problem is <br>
<br>
-------------------<br>
data Experiment p<br>
= Experiment {<br>
e_ext :: (XEOverLit p),<br>
e_val :: Int }<br>
| XExperiment (XXOverLit p)<br>
deriving instance (Data (GhcPass p)) => Data (Experiment (GhcPass p))<br>
<br>
type family XEOverLit x<br>
type family XXOverLit x<br>
<br>
-- The following line works<br>
-- type instance XEOverLit (GhcPass _) = PlaceHolder<br>
<br>
-- The following 3 lines do noe<br>
type instance XEOverLit (GhcPass 'Parsed ) = PlaceHolder<br>
type instance XEOverLit (GhcPass 'Renamed ) = PlaceHolder<br>
type instance XEOverLit (GhcPass 'Typechecked) = PlaceHolder<br>
<br>
type instance XXOverLit (GhcPass _) = PlaceHolder<br>
-------------------<o:p></o:p></p>
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<o:p> </o:p></p>
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Where specifying the type instance with a wild card keeps GHC happy (the XXOverLit case), but specifying for each of the three constructors for pass does not (the XEOverLit case)<o:p></o:p></p>
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<o:p> </o:p></p>
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The exact error message is<o:p></o:p></p>
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<o:p> </o:p></p>
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------------------------------<o:p></o:p></p>
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Experiment.hs:34:1: error:<br>
• Could not deduce (Data (XEOverLit (GhcPass p)))<br>
arising from a use of ‘k’<br>
from the context: Data (GhcPass p)<br>
bound by the instance declaration at Experiment.hs:34:1-69<br>
• In the first argument of ‘k’, namely ‘(z Experiment `k` a1)’<br>
In the expression: ((z Experiment `k` a1) `k` a2)<br>
In an equation for ‘gfoldl’:<br>
gfoldl k z Experiment a1 a2 = ((z Experiment `k` a1) `k` a2)<br>
When typechecking the code for ‘gfoldl’<br>
in a derived instance for ‘Data (Experiment (GhcPass p))’:<br>
To see the code I am typechecking, use -ddump-deriv<br>
|<br>
34 | deriving instance (Data (GhcPass p)) => Data (Experiment (GhcPass p))<br>
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^<br>
------------------------------<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">
<br>
Alan<o:p></o:p></p>
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<o:p> </o:p></p>
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[1] <a href="https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Flpaste.net%2F360017&data=02%7C01%7Csimonpj%40microsoft.com%7C805caf6c0aac49f030ee08d52a785574%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636461618682611590&sdata=VxK0zeEmVsEahWRYMJ6xAG2gZviqGCd%2FHS4DrRXC0dM%3D&reserved=0">
http://lpaste.net/360017</a><o:p></o:p></p>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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