#11316: Too many guards warning causes issues -------------------------------------+------------------------------------- Reporter: NeilMitchell | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler | Version: 7.11 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 gkaracha): Replying to [comment:3 NeilMitchell]:
It seems that if the final guard is {{{| otherwise}}} you could short- circuit the entire analysis pass, since we immediately know what the result is going to be. In such circumstances, emitting a warning about a failed analysis which could be trivially avoided seems unnecessary. It would also provide a simple way for users with "excessively complex" guards to disable the warning, simply add {{{| otherwise -> error "GHC can't cope with this many guards"}}}. No need for the pragma then.
Hmmmm, I disagree with this: The check is not about exhaustiveness only but also about coverage (redundancy) checking, which is by the way the **expensive** part (both problems are NP-Hard but coverage checking appears to trigger exponential behaviour much more often). Hence, having an `otherwise` means that the check is exhaustive but it does not mean that everything is useful: {{{#!hs len x | [] <- x = 0 | (_:ys) <- x = 1 + len ys | otherwise = error "can't happen" }}} Of course the above is exhaustive but it would be nice to see that the last guard is redundant. Since we cannot see that a specific guard is redundant (we can only check for redundant clauses), I see your point. But in principle I always think about coverage/exhaustiveness inseparably. Additionally, in terms of the implementation, I would not like to have so many "special cases", because it becomes really fast unintuitive for the user. What I mean is that I would prefer the warning about too many guards to appear or not independently of the existence of the `otherwise` clause. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11316#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler