#11303: Pattern matching against sets of strings sharing a prefix blows up pattern checker -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: highest | Milestone: 8.0.1 Component: Compiler | Version: 7.11 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: T11303 Blocked By: | Blocking: Related Tickets: #11302 | Differential Rev(s): Phab:D1716, Wiki Page: | Phab:D1719 -------------------------------------+------------------------------------- Comment (by gkaracha): Oh, yes, this is rather expected. I added a note in `deSugar/Check.hs` about the general case: `Note [Translate CoPats]`. As the note says, a `CoPat` is translated as follows: {{{ pat |> co ===> x (pat <- (x |> co)) }}} In the latest commit [changeset:"0acdcf2482d24903b504e6b34fa745ef855ff00d/ghc" 0acdcf24/ghc], I added two cases when translating `CoPat`s: * If `co` is refl we can drop it and do not generate the guard * If `co` is just a hole, we can also drop it. So, as the commit message says, we now generate **less** guards. For data families, this coercion is essential, because changes the representation tycon to the source tycon and I cannot drop it, that is, without changing the type of the pattern. The previous examples had `CoPat`s due to inference but your example above uses data families directly so the workaround does not apply. I can only hope that I will be able to come up with a better translation for `CoPat`s before the release but I do not know how to address this yet. Btw, I think we should move this whole discussion to #11276 since #11303 is rather irrelevant. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11303#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler