#14765: Levity polymorphism panic -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Resolution: | Keywords: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): A few thoughts: * I really don't like having `isUnliftedType` return False for levity polymorphism. Panicking is the right behavior. Anything else might mask bugs. * How would the new `mkCoreApp` work? Would it check its argument for levity polymorphism and then react accordingly? This check is not necessarily cheap, and it would be redundant with other checks for LP. * We could make the new `mkCoreApp` monadic and have it do the only LP check. But then it would need to take the undesugared argument, so that error messages can be at all sensible. That would work in several cases, but sometimes it's not so easy. * You're right that returning `()` is gross. Perhaps returning `undefined` is better. * `dsWhenNoErrs` uses `askNoErrsDs` which does indeed use `tryM`. But there's no great reason `askNoErrsDs` needs to allocate fresh refs for messages, given that messages are ''always'' propagates in `askNoErrsDs`. Perhaps we could just make that more efficient by not using `tryM`. Bottom line: I don't love the current design. But the devil is in the details, and I'm not yet convinced about any new design either. We'd have to try it and see. (I am convinced enough that `undefined` is better than what we have now.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14765#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler