#15493: Elide empty dictionaries -------------------------------------+------------------------------------- Reporter: mnoonan | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Suppose you define a type class `C` with no methods, and use it as a constraint on a function `f :: C => Foo`. GHC's generated Core for `f` currently passes an empty dictionary for `C`. With optimizations, it seems to be true that at use sites, calls to `f` will be replaced with calls to an inner function that skips the typeclass dictionary. But can I count on this optimization always working? Would it possible to get a guarantee that the `C =>` constraint will have no run-time overhead, by dropping it entirely from the generated Core? Or is there a subtlety that prevents this from being sound? For context, I would like to implicitly pass compile-time evidence that various properties hold (e.g. such-and-such a list is non-empty), while retaining a guarantee that the evidence will have no run-time cost. Like this: {{{ newtype Named a name = Named a class Fact p -- Ideally, we'd have a guarantee that the following function -- compiles to exactly the same code as `Prelude.head` head :: Fact (IsCons xs) => (a `Named` xs) -> a head xs = Prelude.head (coerce xs) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15493 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler