#14164: GHC hangs on type family dependency -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: | InjectiveFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones <simonpj@…>): In [changeset:"d6216443c61cee94d8ffc31ca8510a534d9406b9/ghc" d6216443/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="d6216443c61cee94d8ffc31ca8510a534d9406b9" Fix an infinite loop in niFixTCvSubst Trac #14164 made GHC loop, a pretty serious error. It turned out that Unify.niFixTCvSubst was looping forever, because we had a substitution like a :-> ....(b :: (c :: d)).... d :-> ... We correctly recognised that d was free in the range of the substitution, but then failed to apply it "deeeply enough" to the range of the substiuttion, so d was /still/ free in the range, and we kept on going. Trac #9106 was caused by a similar problem, but alas my fix to Trac #9106 was inadequate when the offending type variable is more deeply buried. Urk. This time I think I've fixed it! It's much more subtle than I though, and it took most of a long train journey to figure it out. I wrote a long note to explain: Note [Finding the substitution fixpoint] }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14164#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler