#12860: GeneralizedNewtypeDeriving + MultiParamTypeClasses sends typechecker into an infinite loop -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Yikes that is really bad. At ''least'' we should get a context-stack overflow. But there is more, as I found when investigating: * At the moment if we have {{{ inert: [D] a ~ ty work item [D] a ~ ty }}} we rewrite the work item with the inert to get `[D] ty ~ ty` and then take a series steps to decompose it. It'd be fast to use `unifyDerived` for this case. * The reason we generate two duplicate Derived constraints is also a bit stupid. Suppose we have `[W] C T a`, where class C has some fundeps, and `a` is a skolem of some kind. From this we might generate a derived `[D] a ~ ty`. This kicks out a derived shadow `[D] C T a` from the inerts. We rewrite it to `[D] C T ty`. ''But then we interact it with the `[W] C T a` in the inert set, to make another derived `[D] a ~ ty`. What a waste of effort. But underlying all this is the question of what ''should'' happen here. The problem is: {{{ [W] C a b --> [D] b ~ Foo beta1 (by fundeps, beta1 fresh unification variable) --> [D] C a (Foo beta1) (rewrite) --> [D] C a beta1 (use instance C a b => C a (Foo b)) }}} and now the cycle repeats. With a hand-written instance decl {{{ instance C a b => C a (Foo b) }}} you rightly need `UndecidableInstances`, but it's less clear what to do when trying to guess the instance context; fail in some way I guess. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12860#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler