#10431: EqualityConstraints extension? -------------------------------------+------------------------------------- Reporter: adamgundry | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.11 checker) | 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 adamgundry): 1. Sounds good to me. I think this is probably already the case. The "existentials" flag can be spelled `-XExistentialQuantification -XNoExplicitForAll` and it is indeed useless; here's a poor if obscure error message: {{{ Prelude> set -XExistentialQuantification -XNoExplicitForAll Prelude> data T = forall a . MkT a <interactive>:3:10: Not a data constructor: ‘forall’ Perhaps you intended to use ExistentialQuantification }}} 3. Hmm, this is an interesting point. The current specification is something like this: `GADTs` (or `TypeFamilies`, and perhaps in the future `EqualityConstraints`) is required if pattern matching on a constructor whose context (at the type at which the pattern is being checked) directly includes a nominal equality. Here's another curious example: {{{#!hs data Dict c where MkDict :: c => Dict c f :: Dict (Int ~ Bool) -> Bool f MkDict = True -- requires GADTs g :: Dict a -> Bool g MkDict = True -- doesn't }}} I don't think we should do constraint solving to determine whether an extension is required by a pattern match. So perhaps it's better not to check at all. (But I wouldn't mind terribly if we retain the check as-is.) 4. Fair enough. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10431#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler