#11715: Constraint vs * -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.0.1-rc1 checker) | Resolution: | Keywords: Typeable 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 int-index): I see a few problems with this approach: 1. `RuntimeRep` is used to distinguish things with different run-time representations. I understand why it's sound if things represented equally get different `RuntimeRep`s, but it's not what `RuntimeRep` is for, so those fake inequalities are something to avoid (and you suggest to exploit it). Of course I understand that I'm in no position to tell you what `RuntimeRep` means because you invented it, but as a user I have certain expectations and intuition about what it means, and the name suggests that it means a run-time representation, not some arbitrary distinction which turned out to be convenient! A leaky abstraction. 2. I do think that having `Constraint ~ *` is useful in user code, because right now I have to use `Dict` to go from `Constraint` to `*` and `Given` to go in the opposite direction. It is very inconvenient! 3. We don't get the safe implementation of `reflection` (without `unsafeCoerce`) this way, do we? And if we stop encoding one-element classes as newtypes, the current implementation of `reflection` will break. Making `Constraint` a synonym for `*` sounds very simple and elegant. What are the reasons to pursue a more complicated solution? It seems to me that all of the objections were addressed in this thread. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11715#comment:50 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler