I'm a little out of my depth with rewrite rules, but is there anything
#8767: Add rules involving `coerce` to the libraries -------------------------------------+------------------------------------- Reporter: nomeata | Owner: ekmett Type: task | Status: new Priority: normal | Milestone: 7.12.1 Component: Core Libraries | Version: 7.9 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: 8718 | Test Case: Related Tickets: #2110 | tests/simplCore/should_run/T2110.hs | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by dfeuer): Replying to [comment:27 htebalaka]: preventing rules being associated with a typeclass? Then you could have zero method typeclasses like
{{{ class Functor f => LawfulFunctor f where {-# RULES "fmap/coerce" fmap coerce = coerce #-} }}} and then any datatype that wants the rule just implements the typeclass, which is trivial. It would also address dfeuer's comment about lawfullness in constraints, though it would also generate a lot of noise in documentation (and might inherit the same issue with typeclasses that they can be hard to refactor).
I don't know too much about these things either, but I don't think this will work too well. The problem, as I understand it, is that the `LawfulFunctor` constraint has to be in place at the ''call site'', which will generally not be the case even when the functor is actually an instance of `LawfulFunctor`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8767#comment:28 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler