Re: [GHC] #12102: “Constraints in kinds” illegal family application in instance (+ documentation issues?)

[GHC]

#12102: “Constraints in kinds” illegal family application in instance (+
documentation issues?)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * cc: goldfire (added)


Comment:

 I agree that something is quite fishy is going on here—or perhaps several
 things? The further I dug into this, the more horrified I became. One
 thing I tried was seeing what GHCi thinks of this engimatic `T` type:

 {{{
 $ ghc/inplace/bin/ghc-stage2 --interactive Bug2.hs
 GHCi, version 8.3.20170503: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug2.hs, interpreted )
 Ok, modules loaded: Main.
 λ> :i T
 type role T nominal nominal
 data T (b :: IsTypeLit a ~ 'True) (c :: a) where
   MkNat :: T ('Data.Type.Equality.C:~ ('GHC.Types.Eq# <>)) 42
   MkSymbol :: T ('Data.Type.Equality.C:~ ('GHC.Types.Eq# <>))
                 "Don't panic!"
         -- Defined at Bug2.hs:14:1
 }}}

 ...Oh. Oh my goodness. I don't even know how one is supposed to possibly
 interpret that (perhaps this is related to #13407?).

 Something that's particularly strange is that `T` is reported as having
 //two// type parameters, which is certainly not correct. This might help
 explain all of your attempts to use `T` were met with confusing errors.

 Another thing that perplexes me is—should the definition of `T` as it's
 written in the original example even be accepted? We have:

 {{{#!hs
 data T :: forall a. (IsTypeLit a ~ 'True) => a -> * where ...
 }}}

 If I'm reading this correctly, this would desugar into something like
 this, yes?

 {{{
 data (IsTypeLit a ~ 'True) => T (x :: a) = ...
 }}}

 If so, shouldn't that require `DatatypeContexts`? Also if so, why on earth
 is something that requires `DatatypeContexts` in the users' manual?

 Perhaps this is my inexperience with this feature bleeding through. After
 all, I didn't even know "type constraints" were a thing until I stumbled
 upon this ticket.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12102#comment:1>
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