October 2016 - ghc-tickets

[GHC] #10109: Kinds aren't checked in the coverage condition
by GHC 13 Apr '17

13 Apr '17

#10109: Kinds aren't checked in the coverage condition -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Following on from the debate in #9103 (but you don't need to read that ticket), I found a curiosity. The following module fails to compile, complaining about the unsatisfied coverage condition: {{{ {-# LANGUAGE PolyKinds, MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances #-} module Bug where data Succ a class Add (a :: k1) (b :: k2) (ab :: k3) | a b -> ab, a ab -> b instance (Add a b ab) => Add (Succ a) b (Succ ab) }}} With `-fprint-explicit-kinds`, I get {{{ Bug.hs:9:10: Illegal instance declaration for ‘Add * k * (Succ k a) b (Succ k ab)’ The liberal coverage condition fails in class ‘Add’ for functional dependency: ‘a b -> ab’ Reason: lhs types ‘Succ k a’, ‘b’ do not jointly determine rhs type ‘Succ k ab’ In the instance declaration for ‘Add (Succ a) b (Succ ab)’ }}} But, when I add `k3` to the right-hand side of the first functional dependency (viz. {{{ {-# LANGUAGE PolyKinds, MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances #-} module Bug where data Succ a class Add (a :: k1) (b :: k2) (ab :: k3) | a b -> ab k3, a ab -> b instance (Add a b ab) => Add (Succ a) b (Succ ab) }}} ) I get {{{ [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Ok, modules loaded: Bug. }}} I always assumed that fixing a type in a fundep also fixed its kind. I get the same behavior in 7.8 as in 7.10.1RC2. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10109> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10564: GHC 7.10.2 RC cannot build HList-0.4.0.0
by GHC 13 Apr '17

13 Apr '17

#10564: GHC 7.10.2 RC cannot build HList-0.4.0.0 -------------------------------------+------------------------------------- Reporter: snoyberg | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.10.2 Component: Compiler | Version: 7.10.2-rc1 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: GHC rejects (amd64) | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Another Stackage find: http://lpaste.net/135061 {{{ Data/HList/Record.hs:575:10: Illegal instance declaration for ‘HasFieldM l (r xs) v’ The liberal coverage condition fails in class ‘HasFieldM’ for functional dependency: ‘l r -> v’ Reason: lhs types ‘l’, ‘r xs’ do not jointly determine rhs type ‘v’ In the instance declaration for ‘HasFieldM l (r xs) v’ Data/HList/Record.hs:587:10: Illegal instance declaration for ‘HasField l (Record (Tagged l1 v1 : r)) v’ The liberal coverage condition fails in class ‘HasField’ for functional dependency: ‘l r -> v’ Reason: lhs types ‘l’, ‘Record (Tagged l1 v1 : r)’ do not jointly determine rhs type ‘v’ In the instance declaration for ‘HasField l (Record (Tagged l1 v1 : r)) v’ Data/HList/Record.hs:646:10: Illegal instance declaration for ‘HDeleteAtLabel Record l v v'’ The liberal coverage condition fails in class ‘HDeleteAtLabel’ for functional dependency: ‘l v -> v'’ Reason: lhs types ‘l’, ‘v’ do not jointly determine rhs type ‘v'’ In the instance declaration for ‘HDeleteAtLabel Record l v v'’ Data/HList/Record.hs:723:10: Illegal instance declaration for ‘HUpdateAtLabel2 l v (Tagged l' e : xs) xs'’ The liberal coverage condition fails in class ‘HUpdateAtLabel2’ for functional dependency: ‘l r v -> r'’ Reason: lhs types ‘l’, ‘Tagged l' e : xs’, ‘v’ do not jointly determine rhs type ‘xs'’ In the instance declaration for ‘HUpdateAtLabel2 l v (Tagged l' e : xs) xs'’ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10564> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10570: Too restrictive liberate coverage condition
by GHC 13 Apr '17

13 Apr '17

#10570: Too restrictive liberate coverage condition -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Hello! Let's look at this small example: {{{haskell class ConsByIdx2 x a m cls | x -> m where consByIdx2 :: x -> a -> m cls instance ConsByIdx2 Int a Proxy cls where consByIdx2 _ _ = Proxy }}} It fails to compile with the following error: {{{haskell Illegal instance declaration for ‘ConsByIdx2 Int a Proxy cls’ The liberal coverage condition fails in class ‘ConsByIdx2’ for functional dependency: ‘x -> m’ Reason: lhs type ‘Int’ does not determine rhs type ‘Proxy’ In the instance declaration for ‘ConsByIdx2 Int a Proxy cls’ }}} But Int determines the Proxy in a nice way. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10570> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10797: Kind-level functional dependencies are not resolved properly
by GHC 13 Apr '17

13 Apr '17

#10797: Kind-level functional dependencies are not resolved properly -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Hello! Let's consider such funny class and its instances. {{{ {-# LANGUAGE PolyKinds #-} ... class BaseType (a :: k) (b :: l) | a -> b, k -> l where baseType :: Proxy a -> Proxy b instance BaseType (a :: j -> k) (out :: l) => BaseType (a (t :: j) :: k) (out :: l) }}} The instance does not compile: {{{ Illegal instance declaration for ‘BaseType (a t) out’ The liberal coverage condition fails in class ‘BaseType’ for functional dependency: ‘k -> l’ Reason: lhs type ‘k’ does not determine rhs type ‘l’ In the instance declaration for ‘BaseType (a (t :: j) :: k) (out :: l)’ }}} While the LHS determines the RHS. The problem is that it seems, that GHC doesn't infer the functional dependency of {{{k ~> l}}} from the dependency {{{j -> k ~> l}}}.. I've used a {{{~>}}} arrow to indicate fundeps. Is this a bug or am I missing something? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10797> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12647: Can't capture improvement of functional dependencies
by GHC 13 Apr '17

13 Apr '17

#12647: Can't capture improvement of functional dependencies -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: task | Status: new Priority: normal | Milestone: Component: None | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple FunctionalDependencies | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- [http://www.diku.dk/~paba/pubs/files/serrano15haskell-paper.pdf Type Families with Class, Type Classes with Family] discusses [https://gist.github.com/Icelandjack/5afdaa32f41adf3204ef9025d9da2a70#pdf- instance-improvement instance improvement] with the example {{{#!hs class Collection c e | c -> e where empty :: c add :: e -> c -> c instance Collection [a] a where empty :: [a] empty = [] add :: a -> [a] -> [a] add = (:) }}} I wondered how to express that `x ~ Int` can be deduced from `Collection [Int] x` using improvement, I first attempted using the [https://hackage.haskell.org/package/constraints constraints] machinery: {{{#!hs data Dict c where Dict :: c => Dict c newtype a :- b = Sub (a => Dict b) }}} but I'm not able to construct a term of type `Collection [Int] x :- (Int ~ x)` proving that `Collection [Int] x` entails `Int ~ x`: {{{ ghci> Sub Dict :: Collection [Int] x :- (Int ~ x) <interactive>:52:5: error: • Couldn't match type ‘x1’ with ‘Int’ arising from a use of ‘Dict’ ‘x1’ is a rigid type variable bound by an expression type signature: forall x1. Collection [Int] x1 :- Int ~ x1 at <interactive>:52:13 • In the first argument of ‘Sub’, namely ‘Dict’ In the expression: Sub Dict :: Collection [Int] x :- (Int ~ x) In an equation for ‘it’: it = Sub Dict :: Collection [Int] x :- (Int ~ x) }}} Is this due to overlapping instances or something? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12647> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12704: Check if constraint synonym satisfies functional dependencies
by GHC 13 Apr '17

13 Apr '17

#12704: Check if constraint synonym satisfies functional dependencies -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.1 (Type checker) | 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: -------------------------------------+------------------------------------- When we resolve #12679, it will be possible to implement an abstract type class using a constraint synonym. However, in my implementation, I didn't implement functional dependency checking: {{{ signature H where class F a b | a -> b -- This should be an invalid implementation module H where type F a b = (Eq a, Eq b) }}} The check is annoying fiddly: we have to descend into the constraint synonym, collect all the implied fundeps, and then see if we have the ones we need, so I didn't implement it. Maybe some time. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12704> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12763: Incorrect behavior with empty functional dependencies
by GHC 13 Apr '17

13 Apr '17

#12763: Incorrect behavior with empty functional dependencies -------------------------------------+------------------------------------- Reporter: diatchki | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | 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: -------------------------------------+------------------------------------- This is a corner case, but it would appear that GHC doesn't do proper improvements for classes with "empty" functional dependencies. Consider the following example: {{{#!hs class C a | -> a where m :: a -> () instance C Int f x = m x }}} The inferred type for `f` is `C a => a -> ()`, however I was expecting it to infer `Int -> ()`. The reasoning is as follows: the use of `m` generates a `C a` constraint (`a` is a unification variable). This should have interacted with the `C Int` instance (in a rather odd vacuous sort of way) to generate a derived constraint `a ~ Int`, which should have then instantiated `a` to `Int`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12763> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11372: Loopification does not trigger for IO even if it could
by GHC 11 Apr '17

11 Apr '17

#11372: Loopification does not trigger for IO even if it could -------------------------------------+------------------------------------- Reporter: jscholl | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 (CodeGen) | Keywords: cmm, | Operating System: Unknown/Multiple loopification, code generation | Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The loopification optimization, as I understand it, allows a self- recursive function to jump to a local label instead of the beginning of the function, thus skipping a potential stack check. However, I only observe it triggering for pure functions while IO functions do not get that benefit, even when it would be possible. I discovered this in #8793 after looking into an unexpected speedup by removing the IO context from an otherwise pure loop and while such a loop can simply be changed to a pure version (the {{{IOLoop}}} examples), other functions can not, but the optimization could be applied to them (see {{{MapM.hs}}}). I tried to benchmark the differences between a naive loop in IO and some horrible {{{inlinePerformIO}}} hacks to get the loopification to fire and the "optimized" version performs 3-5% faster on my machine. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11372> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8585: Loopification should omit stack check
by GHC 11 Apr '17

11 Apr '17

#8585: Loopification should omit stack check ------------------------------+-------------------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime performance bug Unknown/Multiple | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | ------------------------------+-------------------------------------------- When we optimize a self-recursive tail call using loopification we don't need to perform a stack check in every loop. This is because a tail recursive function uses constant amount of stack. We should therefore place the loop header after the stack check, but before the heap check (because heap may grow in every call). parcs proposed a patch [https://ghc.haskell.org/trac/ghc/ticket/8275#comment:34 here]. We just need to make sure why it works and add some documentation. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8585> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8871: No-op assignment I64[BaseReg + 784] = I64[BaseReg + 784]; is generated into optimized Cmm
by GHC 11 Apr '17

11 Apr '17

#8871: No-op assignment I64[BaseReg + 784] = I64[BaseReg + 784]; is generated into optimized Cmm ------------------------------------+------------------------------------- Reporter: kgardas | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Other Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- Hello, on SPARC and I also guess on PPC it's possible to get following line in optimized Cmm code: {{{ I64[BaseReg + 784] = I64[BaseReg + 784]; }}} this line is then translated by NCG wasting 5 isns on SPARC at least. Don't know PPC. I'm not sure if this is possible to duplicate this on i386 due to fewer regs. Generally speaking you need to have 32bit target with more regs available. Interesting fact is that such line is not presented in non-optimized Cmm, but is presented in optimized one. Both optimized and non-optimized Cmms attached. Both get from compiling T7507 testcase by stage1 compiler on SPARC: {{{ /home/karel/vcs/ghc-src/ghc-sparc-reg_ncg/inplace/bin/ghc-stage1 -fforce- recomp -dcore-lint -dcmm-lint -dno-debug-output -no-user-package-db -rtsopts -fno-ghci-history -c T7507.hs -O -ddump-opt-cmm > T7507.opt-cmm- sparc-reg-ncg }}} Of course non-opt Cmm is got by -ddump-cmm instead of -ddump-opt-cmm. If you need more simplified testcase, then following code is usable too: {{{ module Main where import Data.Int main = print ( ( 2 ^ 6 ) :: Int64 ) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8871> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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