[GHC] #14111: strange error when using data families with levity polymorphism and unboxed sums and data families

[GHC]

#14111: strange error when using data families with levity polymorphism and unboxed
sums and data families
-------------------------------------+-------------------------------------
        Reporter:  carter            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * keywords:   => TypeFamilies


Comment:

 One important to note is that in this data instance:

 {{{#!hs
 data instance Maybe (x :: TYPE 'UnliftedRep) where
   MaybeSumU :: (# x | (# #) #) -> Maybe x
 }}}

 The `x` in `data instance Maybe (x :: TYPE 'UnliftedRep)` does //not//
 scope over the constructors (this is a quirk of GADTs). Therefore, the
 kind of `x` in `data instance Maybe (x :: TYPE 'UnliftedRep)` doesn't
 necessarily determine the kind of `x` in `MaybeSumU`'s type signature (as
 the `x` in `(# x | (# #) #) -> Maybe x` is bound by an implicit `forall
 x`).

 That being said, I'm still surprised this doesn't typecheck. Here's a
 simpler example of this bug that doesn't involve levity polymorphism or
 `TypeInType`:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 data family   F (a :: k)
 data instance F (a :: Bool) where
   MkF :: F a
 }}}

 {{{
 $ ~/Software/ghc3/inplace/bin/ghc-stage2 --interactive Bug.hs
 GHCi, version 8.3.20170807: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:9:3: error:
     • Data constructor ‘MkF’ returns type ‘F a’
         instead of an instance of its parent type ‘F a’
     • In the definition of data constructor ‘MkF’
       In the data instance declaration for ‘F’
   |
 9 |   MkF :: F a
   |   ^
 Failed, 0 modules loaded.
 λ> :set -fprint-explicit-kinds
 λ> :r
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:9:3: error:
     • Data constructor ‘MkF’ returns type ‘F k a’
         instead of an instance of its parent type ‘F Bool a’
     • In the definition of data constructor ‘MkF’
       In the data instance declaration for ‘F’
   |
 9 |   MkF :: F a
   |   ^
 Failed, 0 modules loaded.
 }}}

 Here's we can see that since `a` appears unadorned in `F a`, its kind
 seems to be inferred as `k` (as given from the data family definition),
 although we'd much rather have it inferred as `Bool`, which you'd expect
 from the instance head.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14111#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler