#15558: Locally handling an empty constraint -------------------------------------+------------------------------------- Reporter: Ericson2314 | Owner: (none) Type: task | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ericson2314): That's better, but I'd still like to remove the type error and the `undefined`: Being able to write `\ @_ -> undefined` would be better, and if we had a `witnessEq :: a ~ b => a :~ b`, I could do `\ @b -> case witnessEq @[b] @a of {}` which would be better still. Best of all with "absurd constraint lambda-case" would be: {{{ g' = \case X -> Bar 0 (\ @_b -> \@case {}) Y -> Bar [] False }}} Which avoids `witnessEq`. Generalizing to all constraints, that's avoidingg some dictionary singleton thing to do the normal term level empty case by doing empty case on the constraint directly. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15558#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler