#1965: Allow unconstrained existential contexts in newtypes -------------------------------------+------------------------------------- Reporter: guest | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 6.8.1 Resolution: | Keywords: 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 sgraf): Note that `data A = A !Int` and `newtype A = A Int` have subtly different surface language semantics when they're pattern matched on. What would `case undefined of A _ -> 1` evaluate to? For newtypes, it's `1` whereas for strict data types this would blow up. I guess what I'm saying is: The suggested lowering must preserve `data` semantics and can never behave the same as actual `newtype`s. Semantically, `data` is `data` and `newtype` is `newtype`. It would get rid of the performance implications, though. See this reddit thread for a longer discussion: https://www.reddit.com/r/haskell/comments/6xri4d/whats_the_difference_betwee... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/1965#comment:38 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler