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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">This is an example of
<a href="https://ghc.haskell.org/trac/ghc/ticket/12088">https://ghc.haskell.org/trac/ghc/ticket/12088</a>.<o:p></o:p></span></p>
<p class="MsoNormal"><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"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">The “type instance T List” declaration actually depends on the “type instance K List” declaration; the latter must be typechecked before the former.
But this dependency is absolutely unclear. There’s a long discussion on the thread. Bottom line: we don’t know a solid automated way to spot this kind of problem, so I think we are going to ask for programmer assistance. In this case, we’d put a “separator”
after the “type instance K List” decl, to explain that it must be done first:<o:p></o:p></span></p>
<p class="MsoNormal"><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"> type instance K List = Type<o:p></o:p></p>
<p class="MsoNormal"> ===========<o:p></o:p></p>
<p class="MsoNormal"> type instance T List = []<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Currently you have to write $(return []) to get the separator, but I think we’ll add a special separator.<o:p></o:p></span></p>
<p class="MsoNormal"><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"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><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"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></a></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"> Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces@haskell.org]
<b>On Behalf Of </b>David Menendez<br>
<b>Sent:</b> 23 September 2016 05:48<br>
<b>To:</b> glasgow-haskell-users@haskell.org Mailing List <Glasgow-haskell-users@haskell.org><br>
<b>Subject:</b> Type families in kind signatures with TypeInType<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">Should the code below work in GHC 8.0.1?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> {-# LANGUAGE TypeInType, TypeFamilies #-}<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> import Data.Kind (Type)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> type family K t :: Type<o:p></o:p></p>
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<p class="MsoNormal"> type family T t :: K t -> Type<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> data List<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> type instance K List = Type<o:p></o:p></p>
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<p class="MsoNormal"> type instance T List = []<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Right now, I get an error like this one:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">min.hs:12:24: error:<o:p></o:p></p>
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<p class="MsoNormal"> • Expected kind ‘K List -> Type’, but ‘[]’ has kind ‘* -> *’<o:p></o:p></p>
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<p class="MsoNormal"> • In the type ‘[]’<o:p></o:p></p>
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<p class="MsoNormal"> In the type instance declaration for ‘T’<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">which is puzzling, since K List -> Type and * -> * should be the same.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Obviously, TypeInType is experimental and incomplete. I’m just wondering if this sort of thing is expected to work, or if I’m doing something not yet supported or never to be supported<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">In particular, the kind signature for T is forall t -> K t -> Type, which looks like DependentHaskell.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-- <o:p></o:p></p>
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<p class="MsoNormal">Dave Menendez <<a href="mailto:dave@zednenem.com" target="_blank">dave@zednenem.com</a>><br>
<<a href="https://na01.safelinks.protection.outlook.com/?url=http:%2f%2fwww.eyrie.org%2f~zednenem%2f&data=01%7C01%7Csimonpj%40microsoft.com%7C740be9e2f66146e93b1308d3e36cdcda%7C72f988bf86f141af91ab2d7cd011db47%7C1&sdata=r83909O38f2c%2feaujupnsW%2fSy23XLouk2unQBGTCB8w%3d" target="_blank">http://www.eyrie.org/~zednenem/</a>><o:p></o:p></p>
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