[GHC] #15493: Elide empty dictionaries

[GHC]

#15493: Elide empty dictionaries
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           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:
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 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>
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