#9082: Unexpected behavior involving closed type families and repeat occurrences of variables ----------------------------------------------+---------------------------- Reporter: haasn | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.8.2 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by haasn): Thanks for the feedback. As I understand it, {{{G}}} breaks this because it actually expands to an infinite type, and hence breaks my assumption that {{{f a ~ a}}} is impossible. This is also the reason why it requires {{{UndecidableInstances}}}. However, it is also my understanding that this type of infinite type family expansion is actually undecidable, in that answering {{{:k! Bad (G Int) (G Int)}}} actually fails to terminate. Do you know if there's an example of such a similarly ill-behaved {{{G}}} that either doesn't require {{{UndecidableInstances}}} or otherwise terminates with an incorrect result/breaks the type system if resolving {{{Bad}}} according to the reasoning outlined earlier? As of now, I don't grok why this would be an actual problem. I'll probably have to look at the papers involved and also #8162 to answer this myself, but I thought I'd quickly ask here first. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9082#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler