#8154: Possible bug in open type familes: Conflicting (a->a) and (a->a->a) instances ----------------------------------------------+---------------------------- Reporter: nh2 | Owner: Type: bug | Status: closed Priority: normal | Milestone: 7.8.1 Component: Template Haskell | Version: 7.7 Resolution: invalid | Keywords: type Operating System: Unknown/Multiple | families Type of failure: GHC rejects valid program | Architecture: Test Case: | Unknown/Multiple Blocking: | Difficulty: Unknown | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Changes (by goldfire): * status: new => closed * resolution: => invalid Comment: Failure in this case is the desired behavior, which is a change from GHC 7.6. Consider {{{ type family Loop a type instance Loop a = Loop a -> Loop a }}} What should `BoundsOf (Loop Bool)` be? Depending on your type family reduction strategy, you could end up choosing either instance in your example, which is very bad. If you want a lot more background, check out section 6 of [http://www.cis.upenn.edu/~eir/papers/2014/axioms/axioms- extended.pdf this draft paper]. But, it seems that implementing your desired behavior is indeed possible with a closed type family: {{{ type family BoundsOf x where BoundsOf (a->a) = Int BoundsOf (a->a->a) = (Int,Int) }}} would probably work just fine for most purposes. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8154#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler