[GHC] #13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings
#13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple PatternSynonyms | Architecture: | Type of failure: Incorrect Unknown/Multiple | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider this program: {{{#!hs {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE ViewPatterns #-} {-# OPTIONS_GHC -Wincomplete-patterns #-} module Bug where data Boolean = F | T deriving Eq pattern TooGoodToBeTrue :: Boolean pattern TooGoodToBeTrue <- ((== T) -> True) where TooGoodToBeTrue = T {-# COMPLETE F, TooGoodToBeTrue #-} catchAll :: Boolean -> Int catchAll F = 0 catchAll TooGoodToBeTrue = 1 }}} This compiles with no warnings with `-Wall`. But if you tweak `catchAll` to add a catch-all case at the end: {{{#!hs {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE ViewPatterns #-} {-# OPTIONS_GHC -Wincomplete-patterns #-} module Bug where data Boolean = F | T deriving Eq pattern TooGoodToBeTrue :: Boolean pattern TooGoodToBeTrue <- ((== T) -> True) where TooGoodToBeTrue = T {-# COMPLETE F, TooGoodToBeTrue #-} catchAll :: Boolean -> Int catchAll F = 0 catchAll TooGoodToBeTrue = 1 catchAll _ = error "impossible" }}} Then if you compile it with `-Wall`, you'll get a very misleading warning: {{{ $ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive Bug.hs -Wall GHCi, version 8.1.20170228: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:17:1: warning: [-Woverlapping-patterns] Pattern match is redundant In an equation for ‘catchAll’: catchAll TooGoodToBeTrue = ... | 17 | catchAll TooGoodToBeTrue = 1 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} I would have expected the warning to be on the `catchAll _ = error "impossible"` case! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | PatternSynonyms 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: | -------------------------------------+------------------------------------- Changes (by simonpj): * cc: gkaracha (added) Comment: George, this is your patch. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | PatternSynonyms 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 gkaracha): Replying to [comment:1 simonpj]:
George, this is your patch.
Hello, I think this is Matthew's patch. If I remember mpickering's implementation correctly, both {`F`, `TooGoodToBeTrue`} and {`F`,`T`} are complete (especially in the module where everything is visible). So all the following combinations are exhaustive: {{{ F, TooGoodToBeTrue -- Using COMPLETE F, _ -- Using the Boolean signature F, _ -- Using COMPLETE }}} Hence, I think this is just non-specified semantics, but not necessarily a bug. Maybe Matthew can shed some more light on this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | PatternSynonyms, | 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: | -------------------------------------+------------------------------------- Changes (by gkaracha): * keywords: PatternSynonyms => PatternSynonyms, PatternMatchWarnings -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | PatternSynonyms, | 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 mpickering): I think this is a bug because the match is not "redundant" as removing it will change the semantics of the function. Similarly if you write {{{ f T = 1 f F = 0 f _ = 3 }}} then removing the F case leaves a complete definition but the correct thing to do is to remove the redundant wildcard clause. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | PatternSynonyms, | 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 gkaracha): Yes, I guess you're right. This seems like the discussion we had on Phab:D2669, but I thought you actually disabled the redundancy check when `COMPLETE` pragmas are involved so that these discrepancies don't show up, with the commit: - Don't warn about redundancy when COMPLETE is involved -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | PatternSynonyms, | 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 mpickering): But that is not the problem. This warning comes from the built-in constructor set. I think there needs to be some more clever augmentation to the intermediate result in the case that a result of `allCompleteMatches` does not contain the constructor which was used to do the lookup. For example, if `ToGoodToBeTrue` is not in `T, F`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: | PatternSynonyms, | 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: | -------------------------------------+------------------------------------- Changes (by dfeuer): * milestone: => 8.4.1 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13363: Wildcard patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: | PatternSynonyms, | 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: | -------------------------------------+------------------------------------- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13363: Wildcard patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: | PatternSynonyms, | PatternMatchWarnings Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: #15554 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: => #15554 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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