#14879: QuantifiedConstraints: Big error message + can't substitute (=>) with a class alias -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedConstraints, wipT2893 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 Iceland_jack): For context this was to encode [https://hackage.haskell.org/package/kan- extensions-5.1/docs/Data-Functor-Yoneda.html Yoneda ((->) a) b] with constraints {{{#!hs newtype Yoneda f b = Yoneda (forall xx. (b -> xx) -> f xx) type Yo a b = Yoneda ((->) a) b lower :: forall a b. (Yo a b) -> (a -> b) lower (Yoneda yoneda) = yoneda (id @b) lift :: (a -> b) -> (Yo a b) lift f = Yoneda (. f) }}} ---- Replying to [comment:1 simonpj]:
Iceland Jack! You are torturing me.
Think of it as enhanced bug reporting In this particular case the fluke does a good thing, it behaves like the `lower` function where instead of applying to the identity function `(id @b)` the constraint solver conjures up the identity constraint `(Implies b b)`.
If we expanded more vigorously, the fluke would happen both times. I'm not sure how hard to work on this.
I'm ok with the looping situation. Would the hypothetical change make `Implies` behave more like `=>`? Making them substitutable ''may'' be a desirable property That being said! The current implementation is very impressive. If something seem unworkable there is surprisingly often some way to guide (trick) GHC into accepting it ([https://gist.github.com/Icelandjack/aeda8e98214cc52c96230af7b8724d25 quantifying over TFs], [https://ghc.haskell.org/trac/ghc/ticket/14878#comment:2 overlap], [https://gist.github.com/Icelandjack/93cf64878e286ed6378adf8fc0e7c200 overlap]). I hope that doesn't change :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14879#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler