#15379: Don't reject user-written instances of KnownNat and friends in hsig files -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: backpack Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4988 Wiki Page: | -------------------------------------+------------------------------------- Comment (by ppk): As mentioned by @simonpj, it would be desirable to actually pinpoint how the dictionaries of `KnownNat` and friends get resolved in the context of backpack. This comment is more to document what I have learned by simply dumping the desugared version. I have attached a test backpack program (attachment:test.bkp) with the output after compiling with `-ddump-ds` and `-ddump-simpl`. The desugared version is avaliable in attachment:out-desugar One can clearly see that an the KnownNat instance is substituted for and the appropriate versions of Util is created [https://ghc.haskell.org/trac/ghc/attachment/ticket/15379/out-desugar#L58 Line 56] and [https://ghc.haskell.org/trac/ghc/attachment/ticket/15379/out- desugar#L92 Line 92]. Although there is no trace of the Util module without the substitution, if we had no mixing-in there will also be a variant of Util with the abstract dictionary for `KnownNat NatType`. So my understanding would be that backpack is going ahead and "inlining" the dictionary when mixing a module that uses a abstract signature with a concrete implementation of that signature. I hope that makes sense. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15379#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler