#12087: Inconsistency in GADTs? -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: RyanGlScott Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: GADTs Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11540 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Replying to [comment:4 simonpj]:
I agree this is possible in principle but it is fiddly in practice. What about {{{ MkF :: Ord a => a -> Eq a => F a }}} which ought to work too.
This isn't exactly what the user wrote, but it's equivalent and serviceable.
Yes but we'll soon have people asking that `:t MkF` prints out the type
Sure, I completely agree. `tcSplitNestedSigmaTys` can handle that case, so `tcSplitNestedLHsSigmaTys` ought to as well. they wrote. We can cross that bridge when we come to it. FWIW, GHC doesn't have a solid track record of preserving nested sigma types for functions either: {{{ λ> let f :: Ord a => Eq a => a -> Bool; f = undefined λ> :type f f :: Ord a => a -> Bool λ> let g :: Ord a => a -> Eq a => a; g = undefined λ> :type g g :: Ord a1 => a2 -> a1 }}} So it (personally) wouldn't bother me to have `:type MkF` print out something slightly different than what I typed. After all, if I'm using nested sigma types in the first place, there's a good chance I know why GHC's displayed type is tweaked (but equivalent). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12087#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler