June 2017 - ghc-tickets

[GHC] #13774: Singletons code fails to typecheck when type signature involving type family is added
by GHC 05 Jun '17

05 Jun '17

#13774: Singletons code fails to typecheck when type signature involving type family is added -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: TypeFamilies | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Yes, I know "singletons" is in the title... but the code isn't //that// scary, I promise. Here's some code which //does// typecheck: {{{#!hs {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} module Bug where data family Sing (a :: k) data Nat = Zero | Succ Nat data instance Sing (b :: Bool) where SFalse :: Sing 'False STrue :: Sing 'True data instance Sing (n :: Nat) where SZero :: Sing 'Zero SSucc :: Sing n -> Sing ('Succ n) type family Not (x :: Bool) :: Bool where Not 'True = 'False Not 'False = 'True sNot :: Sing b -> Sing (Not b) sNot STrue = SFalse sNot SFalse = STrue class PFD a b | a -> b where type L2r (x :: a) :: b instance PFD Bool Nat where type L2r 'False = 'Zero type L2r 'True = 'Succ 'Zero type T2 = L2r 'False class SFD a b | a -> b where sL2r :: forall (x :: a). Sing x -> Sing (L2r x :: b) instance SFD Bool Nat where sL2r SFalse = SZero sL2r STrue = SSucc SZero sT2 = sL2r SFalse }}} It also typechecks if you give `sT2` this particular type signature: {{{#!hs sT2 :: Sing 'Zero sT2 = sL2r SFalse }}} However, if you give it either of these two type signatures: {{{#!hs sT2 :: Sing T2 }}} {{{#!hs sT2 :: Sing (L2r 'False) }}} Then GHC 8.0.1, 8.0.2, 8.2.1, and HEAD will choke: {{{ GHCi, version 8.2.0.20170522: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:46:7: error: • No instance for (SFD Bool k) arising from a use of ‘sL2r’ • In the expression: sL2r SFalse In an equation for ‘sT2’: sT2 = sL2r SFalse | 46 | sT2 = sL2r SFalse | ^^^^^^^^^^^ }}} At this point, I get the urge to yell obscenities at GHC, because there definitely //is// an instance of the form `SFD Bool k` in scope (and moreover, `SFD`'s functional dependency should make sure that `k ~ Nat`). Shouldn't it be using that? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13774> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13761: Can't create poly-kinded GADT with TypeInType enabled, but can without
by GHC 05 Jun '17

05 Jun '17

#13761: Can't create poly-kinded GADT with TypeInType enabled, but can without -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: TypeInType | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Surprisingly, this compiles without `TypeInType`: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE PolyKinds #-} module Works where import Data.Kind data T :: k -> Type where MkT :: T Int }}} But once you add `TypeInType`: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE TypeInType #-} module Bug where import Data.Kind data T :: k -> Type where MkT :: T Int }}} then it stops working! {{{ GHCi, version 8.3.20170516: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:11:12: error: • Expected kind ‘k’, but ‘Int’ has kind ‘*’ • In the first argument of ‘T’, namely ‘Int’ In the type ‘T Int’ In the definition of data constructor ‘MkT’ | 11 | MkT :: T Int | ^^^ }}} This bug is present in GHC 8.0.1, 8.0.2, 8.2.1, and HEAD. What's strange about this bug is that is requires that you write `T` with an explicit kind signature. If you write `T` like this: {{{#!hs data T (a :: k) where MkT :: T Int }}} Then it will work with `TypeInType` enabled. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13761> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13785: Cannot pacify -Wmonomorphism-restriction with nested pattern bindings
by GHC 05 Jun '17

05 Jun '17

#13785: Cannot pacify -Wmonomorphism-restriction with nested pattern bindings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I originally observed this issue in the `parsers` library. Here's a minimal example: {{{#!hs {-# LANGUAGE ScopedTypeVariables #-} {-# OPTIONS_GHC -Wmonomorphism-restriction #-} module Bug where class Monad m => C m where c :: (m Char, m Char) foo :: forall m. C m => m Char foo = bar >> baz where (bar, baz) = c }}} If you compile this with a GHC that supports `-Wmonomorphism-restriction` (GHC 8.0.1 or later), it'll rightfully give this warning: {{{ [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:11:5: warning: [-Wmonomorphism-restriction] • The Monomorphism Restriction applies to the bindings for ‘bar’, ‘baz’ Consider giving a type signature for these binders • In an equation for ‘foo’: foo = bar >> baz where (bar, baz) = c | 11 | (bar, baz) = c | ^^^^^^^^^^^^^^ }}} Naturally, I tried to squelch this warning by adding type signatures for `bar` and `baz`: {{{#!hs foo :: forall m. C m => m Char foo = bar >> baz where bar, baz :: m Char (bar, baz) = c }}} But GHC //still// warns! {{{ [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:12:5: warning: [-Wmonomorphism-restriction] • The Monomorphism Restriction applies to the bindings for ‘bar’, ‘baz’ Consider giving a type signature for these binders • In an equation for ‘foo’: foo = bar >> baz where bar, baz :: m Char (bar, baz) = c | 12 | (bar, baz) = c | ^^^^^^^^^^^^^^ }}} And to make things even more absurd, GHC points out a chunk of code which //has// type signatures :) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13785> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13770: HEAD: Type mentioned in error won't show up in pattern signature
by GHC 05 Jun '17

05 Jun '17

#13770: HEAD: Type mentioned in error won't show up in pattern signature -------------------------------------+------------------------------------- Reporter: heisenbug | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I am encountering a stange problem: {{{ ghc-stage2 T13770.hs -c -fprint-explicit-kinds T13770.hs:29:52: error: ? Could not deduce: (r4 :: GHC.Types.RuntimeRep) ~~ ('GHC.Types.LiftedRep :: GHC.Types.RuntimeRep) from the context: (* ~ *, (f :: *) ~~ (arg -> res :: *)) bound by a pattern with pattern synonym: Fun :: forall k (fun :: k). () => forall (arg :: TYPE r1) (res :: TYPE r2). (k ~ *, (fun :: k) ~~ (arg -> res :: *)) => TypeRep (TYPE r1) arg -> TypeRep (TYPE r2) res -> TypeRep k fun, in a pattern binding in 'do' block at T13770.hs:19:19-35 or from: (* :: *) ~~ (TYPE r1 :: *) bound by a pattern with constructor: Refl :: forall k (a :: k). (:~:) k a a, in a pattern binding in 'do' block at T13770.hs:22:19-22 or from: (* :: *) ~~ (TYPE r2 :: *) bound by a pattern with constructor: Refl :: forall k (a :: k). (:~:) k a a, in a pattern binding in 'do' block at T13770.hs:23:19-22 or from: (TYPE r2 ~ *, (res :: TYPE r2) ~~ (arg1 -> res1 :: *)) bound by a pattern with pattern synonym: Fun :: forall k (fun :: k). () => forall (arg :: TYPE r1) (res :: TYPE r2). (k ~ *, (fun :: k) ~~ (arg -> res :: *)) => TypeRep (TYPE r1) arg -> TypeRep (TYPE r2) res -> TypeRep k fun, in a pattern binding in 'do' block at T13770.hs:27:37-41 ?r4? is a rigid type variable bound by a pattern with pattern synonym: Fun :: forall k (fun :: k). () => forall (arg :: TYPE r1) (res :: TYPE r2). (k ~ *, (fun :: k) ~~ (arg -> res :: *)) => TypeRep (TYPE r1) arg -> TypeRep (TYPE r2) res -> TypeRep k fun, in a pattern binding in 'do' block at T13770.hs:27:37-41 When matching the kind of ?arg1? Expected type: TypeRep * (arg1 -> res1) Actual type: TypeRep (TYPE r2) res ? In the second argument of ?postulate?, namely ?fargrep? In a stmt of a 'do' block: Refl <- postulate vrep fargrep In the expression: do A2 satur _ <- a2 retrep vrep fargrep@Fun {} <- pure retrep HRefl <- fargrep `eqTypeRep` retrep Refl <- postulate vrep fargrep .... | 29 | Refl <- postulate vrep fargrep | ^^^^^^^ }}} Notice how `r4` is mentioned in the error message but none of the signatures refer to it. This is confusing. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13770> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13784: Infinite loop in compiler without undecidableXXX
by GHC 05 Jun '17

05 Jun '17

#13784: Infinite loop in compiler without undecidableXXX -------------------------------------+------------------------------------- Reporter: tysonzero | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code causes the compiler to loop. {{{#!hs {-# LANGUAGE DataKinds, FlexibleContexts, FlexibleInstances, GADTs #-} {-# LANGUAGE KindSignatures, MultiParamTypeClasses, TypeFamilies, TypeOperators #-} module Arithmetic where import Data.Monoid ((<>)) data Product :: [*] -> * where (:*) :: a -> Product as -> Product (a : as) Unit :: Product '[] infixr 5 :* instance Show (Product '[]) where show Unit = "Unit" instance (Show a, Show (Product as)) => Show (Product (a : as)) where show (a :* as) = show a <> " :* " <> show as class Divideable a as where type Divide a as :: [*] divide :: Product as -> (a, Product (Divide a as)) instance Divideable a (a : as) where -- type Divide a (a : as) = as -- Conflicting type family instances, seems like OVERLAPS isn't a thing for type families. divide (a :* as) = (a, as) instance Divideable b as => Divideable b (a : as) where type Divide b (a : as) = a : Divide b as divide (a :* as) = a :* divide as }}} Looks like it is because it is erroneously trying to solve `as ~ Product (Divide a as)` in order to type check `divide`. This bug has been fixed in more recent versions of GHC. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13784> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13783: 更合理的instance Monad []
by GHC 04 Jun '17

04 Jun '17

#13783: 更合理的instance Monad [] -------------------------------------+------------------------------------- Reporter: zaoqi | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Core | Version: 8.0.1 Libraries | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- 使 {{{#!hs join $ map (\y->map (\x->(x,y)) [1..]) [1..] }}} 的结果更合理： https://github.com/zaoqi/zaoqilc/blob/master/featuring/MonadList.hs -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13783> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #7542: GHC doesn't optimize (strict) composition with id
by GHC 04 Jun '17

04 Jun '17

#7542: GHC doesn't optimize (strict) composition with id -------------------------------------+------------------------------------- Reporter: shachaf | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by spacekitteh): Does this still occur in GHC 8.2? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7542#comment:23> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13606: GHCi segfaults on Windows with D3D code
by GHC 03 Jun '17

03 Jun '17

#13606: GHCi segfaults on Windows with D3D code ----------------------------------------+------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.2.1-rc2 Keywords: | Operating System: Windows Architecture: Unknown/Multiple | Type of failure: GHCi crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ----------------------------------------+------------------------------- Warning: this is about as Windows-specific of a bug as it possibly gets, since it requires the use of D3D. I noticed this when trying to run [https://github.com/jwvg0425/d3d11binding examples] from the `d3d11binding` library in GHCi, as they all failed. First, to conjure up the code needed for this: * `Main.hs`: {{{#!hs {-# LANGUAGE CPP #-} {-# LANGUAGE ScopedTypeVariables #-} module Main (main) where import Data.Bits (Bits(..)) import Data.Int (Int32) import Data.Word (Word32) import Foreign.C.String (CString, peekCString, withCString, withCStringLen) import Foreign.Marshal.Alloc (alloca) import Foreign.Ptr (Ptr, castPtr, nullPtr) import Foreign.Storable (Storable(..)) import System.IO (IOMode(..), hGetContents, withFile) #if defined(i386_HOST_ARCH) # define WINDOWS_CCONV stdcall #elif defined(x86_64_HOST_ARCH) # define WINDOWS_CCONV ccall #else # error Unknown mingw32 arch #endif foreign import WINDOWS_CCONV "D3DCompile" c_d3dCompile :: Ptr () -> Word32 -> CString -> Ptr D3DShaderMacro -> Ptr ID3DInclude -> CString -> CString -> D3DCompileFlag -> D3DCompileEffectFlag -> Ptr (Ptr ID3DBlob) -> Ptr (Ptr ID3DBlob) -> IO HRESULT maybePoke :: (Storable a) => Maybe a -> (Ptr a -> IO b) -> IO b maybePoke Nothing proc = proc nullPtr maybePoke (Just m) proc = alloca $ \ptr -> do poke ptr m proc ptr maybeWithCString :: Maybe String -> (CString -> IO a) -> IO a maybeWithCString Nothing proc = proc nullPtr maybeWithCString (Just m) proc = withCString m proc type HRESULT = LONG data ID3DBlob = ID3DBlob data ID3DInclude = ID3DInclue type LONG = Int32 data D3DShaderMacro = D3DShaderMacro { _name :: String , _definition :: String } instance Storable D3DShaderMacro where sizeOf _ = 8 alignment _ = 8 peek ptr = do n <- peekByteOff ptr 0 d <- peekByteOff ptr 4 n' <- peekCString n d' <- peekCString d return $ D3DShaderMacro n' d' poke ptr (D3DShaderMacro n d) = do withCString n $ \n' -> withCString d $ \d' -> do pokeByteOff ptr 0 n' pokeByteOff ptr 4 d' type D3DCompileFlag = Word32 type D3DCompileEffectFlag = Word32 d3dCompileEnableStrictness :: D3DCompileFlag d3dCompileEnableStrictness = shift 1 11 d3dCompile :: String -> Maybe String -> Maybe D3DShaderMacro -> Ptr ID3DInclude -> Maybe String -> String -> [D3DCompileFlag] -> [D3DCompileEffectFlag] -> IO (Either (HRESULT, Ptr ID3DBlob) (Ptr ID3DBlob)) d3dCompile source sourceName defines pInclude entryPoint target compileFlags effectFlags = do withCStringLen source $ \(csource, len) -> withCString target $ \pTarget -> maybeWithCString sourceName $ \pSourceName -> maybePoke defines $ \pDefines -> maybeWithCString entryPoint $ \pEntryPoint -> alloca $ \ppCode -> alloca $ \ppErrorMsgs -> do let sFlag = foldl (.|.) 0 compileFlags let eFlag = foldl (.|.) 0 effectFlags putStrLn "Before d3dCompile" hr <- c_d3dCompile (castPtr csource) (fromIntegral len) pSourceName pDefines pInclude pEntryPoint pTarget sFlag eFlag ppCode ppErrorMsgs putStrLn "After d3dCompile" if hr < 0 then do pErrorMsgs <- peek ppErrorMsgs return $ Left (hr, pErrorMsgs) else do pCode <- peek ppCode return $ Right pCode d3dCompileFromFile :: String -> Maybe String -> Maybe D3DShaderMacro -> Ptr ID3DInclude -> Maybe String -> String -> [D3DCompileFlag] -> [D3DCompileEffectFlag] -> IO (Either (HRESULT, Ptr ID3DBlob) (Ptr ID3DBlob)) d3dCompileFromFile fileName sourceName defines pInclude entryPoint target compileFlags effectFlags = withFile fileName ReadMode $ \handle -> do contents <- hGetContents handle d3dCompile contents sourceName defines pInclude entryPoint target compileFlags effectFlags main :: IO () main = do _vb <- compileShaderFromFile "Triangle.fx" "VS" "vs_4_0" return () compileShaderFromFile :: String -> String -> String -> IO (Ptr ID3DBlob) compileShaderFromFile fileName entryPoint shaderModel = do Right res <- d3dCompileFromFile fileName Nothing Nothing nullPtr (Just entryPoint) shaderModel [d3dCompileEnableStrictness] [] return res }}} * `Triangle.fx` {{{ float4 VS( float4 Pos : POSITION ) : SV_POSITION { return Pos; } float4 PS( float4 Pos : SV_POSITION ) : SV_Target { return float4( 1.0f, 1.0f, 0.0f, 1.0f ); // Yellow, with Alpha = 1 } }}} Make sure that `Triangle.fx` is in the same directory as `Main.hs` when running this program. When compiled, this program works OK: {{{ $ C:\Users\RyanGlScott\Software\ghc-8.2.0.20170404\bin\ghc -lD3DCompiler Main.hs -fforce-recomp [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main.exe ... $ .\Main.exe Before d3dCompile After d3dCompile }}} But with GHCi, it crashes: {{{ $ C:\Users\RyanGlScott\Software\ghc-8.2.0.20170404\bin\runghc -lD3DCompiler Main.hs Before d3dCompile Access violation in generated code when writing 0000000000000000 }}} I ran these tests on GHC 8.2.1, but I've also reproduced this bug in the past on GHC 8.0.2, so I don't think this is a new bug by any means. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13606> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13757: Are you for or against writing "otherwise" as a keyword?
by GHC 03 Jun '17

03 Jun '17

#13757: Are you for or against writing "otherwise" as a keyword? -------------------------------------+------------------------------------- Reporter: vanto | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{otherwise}}} is frequently used and therefore should have been written as a keyword.\\ {{{otherwise}}} was coded in prelude like this {{{otherwise = True}}}\\ So what is the reason that {{{otherwise}}} was not coded as a keyword?\\ \\ To understand you have to go back. It seems that this has been done to minimize the number of keywords.\\ Today it seems surprising. And I wonder why we wrote the conditional expression with {{{if}}}{{{then}}}{{{else}}}? That's three additional keywords.\\ \\ It would have been better to write the conditional expression as in Sasl {{{condition -> exp1 ; exp22}}} (By adapting the operators of course.)\\ Let's go back to {{{otherwise}}} . And take a look at this code.\\ {{{ otherwise :: Bool otherwise = Prelude.otherwise sig :: Integer -> Integer sig n | a = -1 | b = 0 | Main.otherwise = 1 where { a = n < 0; b = n == 0 } z :: Bool z = if Main.otherwise == Prelude.otherwise then True else False main :: IO () main = do { print (sig 0); print (z) } }}} This code works but the compiler result is different by adding or removing the first two lines of code.\\ For me it is urgent that {{{otherwise}}} become a keyword as in Miranda. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13757> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13669: Identifier "Otherwise" in guarded equation can crash a program
by GHC 03 Jun '17

03 Jun '17

#13669: Identifier "Otherwise" in guarded equation can crash a program -------------------------------------+------------------------------------- Reporter: vanto | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- In GHC.Base the identifier {{{otherwise}}} is defined as the value "True" and it helps to make guards more readable.\\ The identifier {{{otherwise}}} is not secure.\\ We can change his property easily and trick the compiler.\\ As written in GHC Base, {{{otherwise = True}}} is perfectible.\\ The problem here is to protect the "otherwise" identifier from uncontrolled access.\\ See the code below and the result of the compilation as well as the output.\\ {{{ import qualified Prelude as P otherwise :: P.Bool otherwise = P.False fun :: (P.Num a, P.Ord a) => a -> [P.Char] fun x | x P.< 0 = "down" | otherwise = "up" main :: P.IO () main = P.print (fun 7) c:\Sourcehs>ghc -Wall otherwise.hs [1 of 1] Compiling Main ( otherwise.hs, otherwise.o ) otherwise.hs:7:1: warning: [-Wincomplete-patterns] Pattern match(es) are non-exhaustive In an equation for `fun': Patterns not matched: _ otherwise.hs:11:17: warning: [-Wtype-defaults] * Defaulting the following constraints to type `P.Integer' (P.Num a0) arising from a use of `fun' at otherwise.hs:11:17-21 (P.Ord a0) arising from a use of `fun' at otherwise.hs:11:17-21 * In the first argument of `P.print', namely `(fun 7)' In the expression: P.print (fun 7) In an equation for `main': main = P.print (fun 7) Linking otherwise.exe ... c:\Sourcehs>otherwise otherwise: otherwise.hs:(7,1)-(8,27): Non-exhaustive patterns in function fun }}} I propose three solutions: 1- Delete {{{otherwise}}} from GHC.Base and use only TRUE.\\ 2- Add {{{otherwise}}} as a reserved word or keyword.\\ 3- Create protected objects in Haskell. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13669> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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