#9103: hackage's type-level-0.2.4 fails to compile -------------------------------------+------------------------------------- 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 | -------------------------------------+------------------------------------- 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)’ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9103#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler