#9161: Pattern synonyms interact badly with data kinds -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: cactus Type: bug | Status: new Priority: lowest | Milestone: Component: Compiler (Type | Version: 7.8.2 checker) | Keywords: renamer, pattern Resolution: | synonyms, data kinds Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: Compile-time | Difficulty: Unknown crash | Blocked By: Test Case: | Related Tickets: Blocking: | -------------------------------------+------------------------------------- Changes (by cactus): * failure: None/Unknown => Compile-time crash * component: Compiler => Compiler (Type checker) * os: Linux => Unknown/Multiple Comment: This one is tricky... One would expect in this scenario roughly the same code path as if you write {{{ {-# LANGUAGE DataKinds #-} data TYPE = CON wrongLift :: CON wrongLift = undefined }}} which BTW results in a reasonable error message: {{{ Expected a type, but ‘CON’ has kind ‘TYPE’ In the type signature for ‘wrongLift’: wrongLift :: CON }}} However, since pattern synonyms are internally handled as binds, not `TyClDecl`s, they are not yet in scope during kind checking (which is done by `tcTyClsInstDecls`) - hence the crash. How do we even get to trying to kindcheck them? `OccName.demoteOccName` handles data constructor names and type constructor names the same way, since we don't differentiate between the two there. So I think our best bet would be to have a bespoke internal kind for pattern synonyms, inject pattern synonyms at that kind before doing kind checking, and then barf on types of that kind with a meaningful error message. Simon, does that sound like a good plan of action? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9161#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler