[GHC] #13965: COMPLETE sets nerf redundant pattern-match warnings
#13965: COMPLETE sets nerf redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Keywords: | Operating System: Unknown/Multiple PatternSynonyms, | PatternMatchWarnings | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If I give redundant patterns for a datatype with a `COMPLETE` set, then GHC doesn't warn me at all. Here's an example: {{{#!hs {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE ViewPatterns #-} module Bug (Boolean(F, TooGoodToBeTrue), catchAll) 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 F = 2 }}} GHC thinks this is fine and dandy: {{{ $ /opt/ghc/8.2.1/bin/ghci Bug.hs -Wall GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Ok, 1 module loaded. }}} This problem also persists across modules: {{{#!hs module Foo where import Bug catchAll2 :: Boolean -> Int catchAll2 F = 0 catchAll2 TooGoodToBeTrue = 1 catchAll2 F = 2 }}} {{{ $ /opt/ghc/8.2.1/bin/ghci Foo.hs -Wall GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 2] Compiling Bug ( Bug.hs, interpreted ) [2 of 2] Compiling Foo ( Foo.hs, interpreted ) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13965 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13965: COMPLETE sets nerf redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: | PatternSynonyms, | PatternMatchWarnings Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by ckoparkar): * cc: mpickering (added) Comment: It looks like this was by design when the `COMPLETE` pragma was added in Phab:D2669. {{{ #!diff compiler/deSugar/Check.hs @@ -1360,8 +1455,8 @@ wrapUpTmState (residual, (_, subst)) = (residual, flattenPmVarEnv subst) dsPmWarn :: DynFlags -> DsMatchContext -> PmResult -> DsM () dsPmWarn dflags ctx@(DsMatchContext kind loc) pm_result = when (flag_i || flag_u) $ do - let exists_r = flag_i && notNull redundant - exists_i = flag_i && notNull inaccessible + let exists_r = flag_i && notNull redundant && onlyBuiltin + exists_i = flag_i && notNull inaccessible && onlyBuiltin exists_u = flag_u && notNull uncovered when exists_r $ forM_ redundant $ \(L l q) -> do putSrcSpanDs l (warnDs (Reason Opt_WarnOverlappingPatterns) @@ -1373,7 +1468,8 @@ dsPmWarn dflags ctx@(DsMatchContext kind loc) pm_result }}} Relevant comment: {{{compiler/deSugar/Check.hs}}} {{{ #!haskell data PartialResult = PartialResult { presultProvenance :: Provenance -- keep track of provenance because we don't want -- to warn about redundant matches if the result -- is contaminated with a COMPLETE pragma ...} }}} Do we want to change this? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13965#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13965: COMPLETE sets nerf redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: | PatternSynonyms, | PatternMatchWarnings Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by ckoparkar): * cc: ckoparkar (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13965#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13965: COMPLETE sets nerf redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: | PatternSynonyms, | PatternMatchWarnings 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 mpickering): It's this way because removing a "redudant" match arising from a `COMPLETE` pragma can change what a function does. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13965#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13965: COMPLETE sets nerf redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: | PatternSynonyms, | PatternMatchWarnings 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 RyanGlScott): Replying to [comment:3 mpickering]:
It's this way because removing a "redudant" match arising from a `COMPLETE` pragma can change what a function does.
Really? What is an example of this? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13965#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13965: COMPLETE sets nerf redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: | PatternSynonyms, | PatternMatchWarnings 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 mpickering): {{{ data T = A | B {-# COMPLETE A #-} foo A = 5 foo B = 6 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13965#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13965: COMPLETE sets nerf redundant pattern-match warnings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: | PatternSynonyms, | PatternMatchWarnings 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 RyanGlScott): I think this comparison is somewhat unfair. For one thing, this isn't even a situation where the `COMPLETE` set would kick in, since GHC would pick the original set of constructors over the `COMPLETE` set. Removing the "redundant" match on `B` wouldn't introduce any warnings either, but that is correct behavior, since now GHC uses the `COMPLETE` set. The original program is of a different nature, since the only option it has is a `COMPLETE` set (as evidenced by `module Foo`). Moreover, removing the redundant match on `F` doesn't change the fact that GHC is using the same `COMPLETE` set. This is why I argue that GHC's warning behavior on the original program is mistaken, since the second match on `F` is truly redundant, even after factoring in the semantics of `COMPLETE` sets. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13965#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
participants (1)
-
GHC