
#13772: Cannot put HasCallStack on instances -------------------------------------+------------------------------------- Reporter: augustss | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: 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 simonpj): {{{ foo :: Monoid a => (a,a) foo = (mempty, mempty) bar :: Tree a bar = fst foo }}} You say you want a `HasCallStack` constraint just on the `Tree` instance of `mempty`. But I don't think you can do that. The call site of `mempty` is in `foo`, which is polymorhpic, works for any instance. Only in `bar` do we beome specific to the `Tree` instance. It'd be fine if you wanted to say {{{ class Monoid a where mempty :: HasCallStack => a ... }}} but I doubt you do. I don't see how to achieve what you want. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13772#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler