#9371: Overlapping type families, segafult ----------------------------------------+---------------------------------- Reporter: pingu | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 (amd64) Type of failure: Runtime crash | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | ----------------------------------------+---------------------------------- Comment (by goldfire): Replying to [comment:5 simonpj]:
Moreover, doesn't your example work perfectly well without eta
{{{ IdentityT [] Int ~R IdentityT MyList Int }}} Now, using the `Coercible` instances in Fig 2 of [http://research.microsoft.com/en- us/um/people/simonpj/papers/ext-f/coercible.pdf the paper], we can reduce
{{{ [Int] ~R MyList Int }}} and that is true with the non-eta-reduced axiom.
If `IdentityT`'s data constructor was not in scope, then indeed the
reduction? To typecheck the `deriving instance` we need that to program will still typecheck -- but arguably doing so exposes something about the representation of the newtype, harming abstraction. You certainly couldn't have written it be hand. I hadn't considered the newtype-unwrapping axiom when writing the example. You're right -- we don't need eta reduction. But, if `IdentityT` were a ''data''type instead of a newtype, we would, without further changes to the example. The "harming abstraction" note is exactly the debate about whether or not we should be able to coerce `Map String Int` to `Map String Age` without `Map`'s constructor being in scope.
It all seems quite debatable to me. Do we really want a significant
cluster of complexity in the implementation, to implement an un-documented feature, which no one is asking for, and whose very existence is debatable? It still makes me nervous to reduce expressiveness -- I just don't want users of 7.10 to suddenly lose something, post here that they need it back, and then have it restored for 7.12, leaving a lot of CPP in their code. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9371#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler