#15909: prepareAlts does not take into account equalities which are in scope -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.3 Component: Compiler | Version: 8.6.2 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 simonpj): Reasoning about equalities is the business of the typechecker, and the simplifier doesn't do much. But it does do some things; e.g. it tries to reduce type-family applications. It would be reasonable to try to make it do more; and indeed to support the pattern-match overlap stuff we have an API for the typechecker that can be called externally. I don't know if it's the ''right'' API for this purpose. To make this work we'd have to accumulate the "givens" as we go down. Not too hard. I'd be inclined to do this in a separate pass, NOT the simplifier which has too much else to do. Just possibly the occurrence analyser, though I worry about slowing it down. Another tricky point is that I'd like to be able to prove that every case is exhaustive, and a Lint-checkable property. But if I later remove the second parameter to f2 (it is dead) then it won't be provable anymore. This problem is more basic than fancy equalities. Consider {{{ case x of Red -> e1 DEFAULT -> let v = case x of Red -> e2 Blue -> e3 Green -> e4 in blah }}} we can prune the `Red` branch out of the `case` in the RHS of `v`. But now imagine floating the `v`-binding: {{{ let v = case x of Blue -> e3 Green -> e4 in case x of Red -> e1 DEFAULT -> blah }}} This is actually still correct, but it's not ''obviously'' correct. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15909#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler