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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">I think just add a new constructor to EvTerm.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Yes, it’s special-purpose, but the *<b>solver*
</b>is special-purpose too. And it does mean that we know exactly what forms of evidence we can generate!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon</span><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<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"> Iavor Diatchki [mailto:iavor.diatchki@gmail.com]
<br>
<b>Sent:</b> 07 February 2015 20:11<br>
<b>To:</b> Simon Peyton Jones; ghc-devs@haskell.org<br>
<b>Subject:</b> Question about implementing `Typeable` (with kinds)<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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Hello,<o:p></o:p></p>
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<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">
I started adding custom solving for `Typeable` constraints, to work around the problem where kind parameters were missing from the representation of types. <o:p></o:p></p>
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<o:p> </o:p></p>
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The idea is as follows:<o:p></o:p></p>
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<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">
1. Add a new filed to `<span style="font-family:"Courier New"">TypeRep</span>` that remembers _kind_ parameters:<o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Courier New"">TypeRep Fingerprint TyCon [TypeRep]{-kinds-} [TypeRep]{-types-}</span><o:p></o:p></p>
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<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">
2. Modify the constraint solver, to solve constraints like this:<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">
- Kind-polymorphic type constructors don't get `<span style="font-family:"Courier New"">Typeable</span>` instances on their own<o:p></o:p></p>
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- GHC can solve `<span style="font-family:"Courier New"">Typeable</span>` constraints on _<i>concrete uses</i>_ of polymorphic type constructors.<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">
More precisely, GHC can solve constraints of the form `<span style="font-family:"Courier New"">Typeable k (TC @ ks)</span>`, as long as:<o:p></o:p></p>
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(1) `<span style="font-family:"Courier New"">k</span>` is not a forall kind,<o:p></o:p></p>
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(2) the `<span style="font-family:"Courier New"">ks</span>` are all concrete kinds (i.e., they have no free kind variables).<o:p></o:p></p>
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<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">
This all seems fairly straight-forward, but I got stuck on the actual implementation, in particular:<o:p></o:p></p>
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<o:p> </o:p></p>
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<b>what `</b><b><span style="font-family:"Courier New"">EvTerm</span>` should I use when discharging a `</b><b><span style="font-family:"Courier New"">Typeable</span>` constraint?</b><o:p></o:p></p>
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<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">
I can create a an `<span style="font-family:"Courier New"">HsSyn</span>` value for the required method (i.e., a function of type `<span style="font-family:"Courier New"">Proxy# t -> TypeRep</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">
I can also cast this into a `<span style="font-family:"Courier New"">Typeable</span>` dictionary value.<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">
The issue is that I am left with an `<span style="font-family:"Courier New"">HsSyn</span>` expression, and not an `<span style="font-family:"Courier New"">EvTerm</span>`.<o:p></o:p></p>
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<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">
So is there a way to treat an arbitrary expression as an `EvTerm`?<o:p></o:p></p>
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<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">
In the implementation of the type-lits, I just added custom evidence, but this does not scale well (also, in that case the evidence is just a simple value, while here<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">
it is a bit more complex).<o:p></o:p></p>
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<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">
Suggestions would be most appreciated!<o:p></o:p></p>
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<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">
-Iavor<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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<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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