#15305: Erroneous "non-exhaustive pattern match" using nested GADT with strictness annotation -------------------------------------+------------------------------------- Reporter: jkoppel | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.4.3 checker) | Keywords: Resolution: | PatternMatchWarnings 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 RyanGlScott): I tried thinking of an (ad hoc) algorithm to check for this sort of thing. This is the closest I could get: 1. In `mkOneConFull` (which generates pattern-matching information for each constructor), record the type and strictness of each constructor's fields. 2. When checking if a match on that constructor is reachable, perform the following additional checks: i. Check if all fields are strict. ii. Check if each field's type is uninhabitable. If both (i) and (ii) are true, then deem the entire constructor to be unreachable. I think this would work for the original program, but not the program in comment:6 {{{#!hs data Abyss = MkAbyss !Abyss stareIntoTheAbyss :: Abyss -> a stareIntoTheAbyss a = case a of {} }}} Why? In order to check that a match on `(MkAbyss _)` is unreachable, we need to check: i. That the field of `MkAbyss` is strict. (This is the case.) ii. That the type of the field of `MkAbyss` (`Abyss`) is uninhabitable. But in order to check that `Abyss` in uninhabitable, we need to check the `MkAbyss` constructor again, which requires repeating this algorithm. Now we've entered an infinite loop! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15305#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler