#12390: List rules for `Coercible` instances -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: -------------------------------------+------------------------------------- `:info` on [https://downloads.haskell.org/~ghc/7.8.2/docs/html/libraries/base-4.7.0.0 /Data-Coerce.html#t:Coercible Data.Coerce.Coercible] doesn't display any instances, it's a special class so it should be special-cased {{{ ghci> :i Coercible type role Coercible representational representational class a ~R# b => Coercible (a :: k) (b :: k) -- Defined in ‘GHC.Types’ }}} The base case would be easy to display: {{{#!hs instance Coercible a a -- Generated on the fly }}} but it's trickier to describe the … instance succinctly: {{{#!hs instance Coercible b b' => Coercible (D a b c) (D a b' c') }}} same for newtypes. ---- Maybe it's better to list concrete examples {{{#!hs coerce @(Reader _ _) :: Reader a b -> (a -> Identity b) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12390 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler