#13775: Type family expansion is too lazy, allows accepting of ill-typed terms -------------------------------------+------------------------------------- Reporter: fizruk | Owner: diatchki Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.2 checker) | Keywords: Resolution: | CustomTypeErrors Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
I think that this is more a question about how type functions work, rather than TypeError specifically.
No, I don't think it is. GHC ''never'' reports an error simply becuase it tries to evaluate `TypeError ..`. Rather, that call is stuck. That in turn may mean that some constraint can't be solved. And then, when reporting unsolved constraints, GHC looks for any nested calls to `TypeError` and reports them rather than the constraint itself. You wrote this code! It's nothing to do with how much reduction takes place, IMHO. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13775#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler