#9063: Default associated type instances are too general -------------------------------------+------------------------------------ Reporter: goldfire | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by Simon Peyton Jones <simonpj@…>): In [changeset:"9b8ba62991ae22420a0c4486127a3b22ee7f22bd/ghc"]: {{{ #!CommitTicketReference repository="ghc" revision="9b8ba62991ae22420a0c4486127a3b22ee7f22bd" Entirely re-jig the handling of default type-family instances (fixes Trac #9063) In looking at Trac #9063 I decided to re-design the default instances for associated type synonyms. Previously it was all jolly complicated, to support generality that no one wanted, and was arguably undesirable. Specifically * The default instance for an associated type can have only type variables on the LHS. (Not type patterns.) * There can be at most one default instances declaration for each associated type. To achieve this I had to do a surprisingly large amount of refactoring of HsSyn, specifically to parameterise HsDecls.TyFamEqn over the type of the LHS patterns. That change in HsDecls has a (trivial) knock-on effect in Haddock, so this commit does a submodule update too. The net result is good though. The code is simpler; the language specification is simpler. Happy days. Trac #9263 and #9264 are thereby fixed as well. }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9063#comment:9> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler