[GHC] #9103: hackage's type-level-0.2.4 fails to compile

[GHC]

#9103: hackage's type-level-0.2.4 fails to compile
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        Reporter:  slyfox            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.8.2
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:  #8634             |
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Comment (by jstolarek):

 I've just run into this problem trying to run this toy example from
 [[https://wiki.haskell.org/Type_arithmetic|Haskell Wiki]]:

 {{{#!hs
 data Zero
 data Succ a

 class Add a b ab | a b -> ab, a ab -> b
 instance Add Zero b b
 instance (Add a b ab) => Add (Succ a) b (Succ ab)
 }}}
 {{{
 Illegal instance declaration for ‘Add (Succ a) b (Succ ab)’
   The liberal coverage condition fails in class ‘Add’
     for functional dependency: ‘a b -> ab’
   Reason: lhs types ‘Succ a’, ‘b’
     do not jointly determine rhs type ‘Succ ab’
 In the instance declaration for ‘Add (Succ a) b (Succ ab)’
 }}}
 Interestingly, replacing empty data declarations of `Zero` and `Succ a`
 with data kinds makes the code compile:
 {{{#!hs
 data Nat = Zero | Succ Nat

 class Add a b ab | a b -> ab, a ab -> b
 instance Add Zero b b
 instance (Add a b ab) => Add (Succ a) b (Succ ab)
 }}}
 Why with empty data types this is wrong but with promoted types everything
 is fine? Also, HEAD complains about redundant constraint:
 {{{
 Redundant constraint: Add a b ab
 In the instance declaration for ‘Add ('Succ a) b ('Succ ab)’
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9103#comment:7>
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