November 2016 - ghc-tickets

[GHC] #12768: 8.0.2 derives invalid code when class method is constrained by itself
by GHC 01 Aug '17

01 Aug '17

#12768: 8.0.2 derives invalid code when class method is constrained by itself -------------------------------------+------------------------------------- Reporter: jophish | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | 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: -------------------------------------+------------------------------------- GHC 8.0.1 is able to compile this without a problem and doesn't require FlexibleContexts. {{{#!hs {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE ConstrainedClassMethods #-} module A where class C m where foo :: C m => m () newtype N m a = N (m a) deriving C }}} Compare the output of 8.0.1, 8.0.2 and 8.1. I turned on -fdefer-type- errors in order for -ddump-deriv to work. {{{ $ ghc --version The Glorious Glasgow Haskell Compilation System, version 8.0.1 $ ghc -ddump-deriv A.hs [1 of 1] Compiling A ( A.hs, A.o ) ==================== Derived instances ==================== Derived instances: instance A.C m_aNK => A.C (A.N m_aNK) where A.foo = GHC.Prim.coerce (A.foo :: m_ap1 ()) :: A.N m_ap1 () GHC.Generics representation types: }}} {{{ $ ghc --version The Glorious Glasgow Haskell Compilation System, version 8.0.2 $ ghc A.hs [1 of 1] Compiling A ( A.hs, A.o ) A.hs:10:12: error: • Couldn't match type ‘m’ with ‘N m’ arising from the coercion of the method ‘foo’ from type ‘C m => m ()’ to type ‘C (N m) => N m ()’ ‘m’ is a rigid type variable bound by the deriving clause for ‘C (N m)’ at A.hs:10:12 • When deriving the instance for (C (N m)) $ ghc -ddump-deriv -fdefer-type-errors A.hs [1 of 1] Compiling A ( A.hs, A.o ) ==================== Derived instances ==================== Derived instances: instance A.C m_awm => A.C (A.N m_awm) where A.foo = GHC.Prim.coerce @(A.C m_ap0 => m_ap0 ()) @(A.C (A.N m_ap0) => A.N m_ap0 ()) A.foo GHC.Generics representation types: A.hs:11:12: warning: [-Wdeferred-type-errors] • Couldn't match type ‘m’ with ‘N m’ arising from a use of ‘GHC.Prim.coerce’ ‘m’ is a rigid type variable bound by the instance declaration at A.hs:11:12 • In the expression: GHC.Prim.coerce @(C m => m ()) @(C (N m) => N m ()) foo In an equation for ‘foo’: foo = GHC.Prim.coerce @(C m => m ()) @(C (N m) => N m ()) foo When typechecking the code for ‘foo’ in a derived instance for ‘C (N m)’: To see the code I am typechecking, use -ddump-deriv In the instance declaration for ‘C (N m)’ • Relevant bindings include foo :: N m () (bound at A.hs:11:12) }}} There's no '8.0.2' version to report this against so I've chosen 8.1. GHC 8.1 gives very similar results: {{{ $ ghc --version The Glorious Glasgow Haskell Compilation System, version 8.1.20160930 $ ghc A.hs [1 of 1] Compiling A ( A.hs, A.o ) A.hs:11:12: error: • Couldn't match type ‘m’ with ‘N m’ arising from the coercion of the method ‘foo’ from type ‘C m => m ()’ to type ‘C (N m) => N m ()’ ‘m’ is a rigid type variable bound by the deriving clause for ‘C (N m)’ at A.hs:11:12 • When deriving the instance for (C (N m)) $ ghc -ddump-deriv -fdefer-type-errors A.hs [1 of 1] Compiling A ( A.hs, A.o ) ==================== Derived instances ==================== Derived instances: instance A.C m_awK => A.C (A.N m_awK) where A.foo = GHC.Prim.coerce @(A.C m_app => m_app ()) @(A.C (A.N m_app) => A.N m_app ()) A.foo GHC.Generics representation types: A.hs:11:12: warning: [-Wsimplifiable-class-constraints] The constraint ‘C (N m)’ matches an instance declaration instance C m => C (N m) -- Defined at A.hs:11:12 This makes type inference for inner bindings fragile; either use MonoLocalBinds, or simplify it using the instance A.hs:11:12: warning: [-Wdeferred-type-errors] • Couldn't match type ‘m’ with ‘N m’ arising from a use of ‘GHC.Prim.coerce’ ‘m’ is a rigid type variable bound by the instance declaration at A.hs:11:12 • In the expression: GHC.Prim.coerce @(C m => m ()) @(C (N m) => N m ()) foo In an equation for ‘foo’: foo = GHC.Prim.coerce @(C m => m ()) @(C (N m) => N m ()) foo When typechecking the code for ‘foo’ in a derived instance for ‘C (N m)’: To see the code I am typechecking, use -ddump-deriv In the instance declaration for ‘C (N m)’ • Relevant bindings include foo :: N m () (bound at A.hs:11:12) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12768> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 15

[GHC] #12417: API annotations for unboxed sums needs reworking
by GHC 01 Aug '17

01 Aug '17

#12417: API annotations for unboxed sums needs reworking -------------------------------------+------------------------------------- Reporter: osa1 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: -------------------------------------+------------------------------------- API annotations for unboxed sums are not entirely correct. As far as I understand, some of the bars (`|`) in the concrete syntax are not properly attached to any of the AST nodes. Some discussion can be seen in [https://phabricator.haskell.org/D2259#69809 the Phab page]. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12417> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 6

[GHC] #12478: Template Haskell support for unboxed sums
by GHC 01 Aug '17

01 Aug '17

#12478: Template Haskell support for unboxed sums -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: feature | Status: new request | Priority: normal | Milestone: 8.2.1 Component: Template | Version: 8.1 Haskell | Keywords: | Operating System: Unknown/Multiple TemplateHaskell | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Currently, you can't use unboxed sums with Template Haskell. This makes me sad. Patch incoming. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12478> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 6

[GHC] #11995: Can't infer type
by GHC 31 Jul '17

31 Jul '17

#11995: Can't infer type -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | 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: -------------------------------------+------------------------------------- {{{#!hs import Data.Kind data NP :: forall k. (ĸ → Type) → ([k] → Type) where Nil :: NP f '[] (:*) :: f x → NP f xs → NP f (x:xs) newtype K a b = K a deriving Show unK (K a) = a h'collapse :: NP (K a) xs -> [a] h'collapse = \case Nil -> [] K x:*xs -> x : h'collapse xs }}} if we replace `xs` by an underscore: {{{ tJN0.hs:13:29-30: error: … • Could not deduce: (xs :: [ĸ]) ~~ ((':) ĸ x xs :: [ĸ]) from the context: ((k :: *) ~~ (ĸ :: *), (t :: [k]) ~~ ((':) ĸ x xs :: [ĸ])) bound by a pattern with constructor: :* :: forall k (f :: k -> *) (x :: k) (xs :: [k]). f x -> NP k k f xs -> NP k k f ((':) k x xs), in a case alternative at /tmp/tJN0.hs:13:3-9 ‘xs’ is a rigid type variable bound by a pattern with constructor: :* :: forall k (f :: k -> *) (x :: k) (xs :: [k]). f x -> NP k k f xs -> NP k k f ((':) k x xs), in a case alternative at /tmp/tJN0.hs:13:3 Expected type: NP ĸ k (K ĸ a) t Actual type: NP ĸ ĸ (K ĸ a) xs • In the first argument of ‘h'collapse’, namely ‘xs’ In the second argument of ‘(:)’, namely ‘h'collapse xs’ In the expression: x : h'collapse xs • Relevant bindings include xs :: NP ĸ ĸ (K ĸ a) xs (bound at /tmp/tJN0.hs:13:8) h'collapse :: NP ĸ k (K ĸ a) t -> [a] (bound at /tmp/tJN0.hs:11:1) Compilation failed. }}} Should it not be able to infer that? The Glorious Glasgow Haskell Compilation System, version 8.1.20160419 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11995> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 5

[GHC] #12878: Use gold linker by default if available on ELF systems
by GHC 31 Jul '17

31 Jul '17

#12878: Use gold linker by default if available on ELF systems -------------------------------------+------------------------------------- Reporter: dobenour | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Driver | Version: 8.0.1 Keywords: | Operating System: Linux Architecture: | Type of failure: Compile-time Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- `ld.gold` is both faster and less buggy than `ld.bfd`, but only works on ELF. On ELF systems, we should thus prefer `ld.gold` when it is installed, even when `ld.bfd` is supported (as it is on x86). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12878> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 2

[GHC] #9020: Massive blowup of code size on trivial program
by GHC 31 Jul '17

31 Jul '17

#9020: Massive blowup of code size on trivial program ------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- The test `simplCore/should_compile/simpl015` was leading to massive compile times. This is weird: it looks like: {{{ main = do { return () ; return () ... hundreds more times ... ; return () } }}} It turns out that the cause was over-eager eta-expansion. Each `return ()` gives a top-level PAP thus {{{ lvl17 :: IO () lvl17 = returnIO () }}} But these definitions are then eta-expanded, thus: {{{ lvl17 = ((\eta. returnIO () |> sym g) eta) |> g where g :: State# RealWorld -> (# State# RealWorld, () #) ~ IO () }}} Now in general it makes sense to eta-expand through newtypes (as is done here), because it exposes more lambadas. But it really doesn't make sense to eta-expand a PAP like this. Fortunately the fix is easy: use `exprArity` rather than `manifestArity` in `SimplUtils.tryEtaExpandRhs`. The effect on `simpl015` is dramatic: compiler allocation drops from 6.6G to 812M; and residency drops from 1.8G to 45M. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9020> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 13

[GHC] #12018: Equality constraint not available in pattern type signature (GADTs/ScopedTypeVariables)
by GHC 28 Jul '17

28 Jul '17

#12018: Equality constraint not available in pattern type signature (GADTs/ScopedTypeVariables) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: lowest | Milestone: Component: Compiler | Version: 7.10.3 (Type checker) | Keywords: GADTs, | Operating System: Unknown/Multiple ScopedTypeVariables | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs data Exp a where Tru :: Ty Bool eval :: Exp a -> a eval Tru = True }}} Works fine, as does {{{#!hs eval :: Exp a -> a eval (Tru :: Exp _) = True }}} But {{{#!hs eval :: Exp a -> a eval (Tru :: Exp Bool) = True }}} doesn't, is this an intended design of GADTs/ScopedTypeVariables that the type equality constraint isn't in scope in the type signature of the pattern match, I would like to match on an existential type in my own code: {{{#!hs compile (ArrIx arr index :: Exp (Sca elt)) = do ... }}} It can be worked around by writing: {{{#!hs eval a@Tru = case a :: Exp Bool of }}} but for my own code it seems I must write {{{#!hs compile uuu@(ArrIx arr index :: Exp a) = do case uuu :: (a ~ Sca elt) => Exp (Sca elt) of }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12018> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 11

[GHC] #12369: data families shouldn't be required to have return kind *, data instances should
by GHC 27 Jul '17

27 Jul '17

#12369: data families shouldn't be required to have return kind *, data instances should -------------------------------------+------------------------------------- Reporter: ekmett | Owner: Type: feature | Status: new request | Priority: normal | Milestone: 8.2.1 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: -------------------------------------+------------------------------------- I'd like to be able to define {{{#!hs {-# language PolyKinds, KindSignatures, GADTs, TypeFamilies #-} data family Fix :: (k -> *) -> k newtype instance Fix f = In { out :: f (Fix f) } }}} But currently this is disallowed: {{{ Fix.hs:2:1: error: • Kind signature on data type declaration has non-* return kind (k -> *) -> k • In the data family declaration for ‘Fix’ }}} Ultimately I think the issue here is that data __instances__ should be required to end in kind *, not the families, but this generalization didn't mean anything until we had `PolyKinds`. As an example of a usecase, with the above, I could define a bifunctor fixed point such as {{{#!hs newtype instance Fix f a = In2 { out2 :: f (Fix f a) a } }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12369> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 15

[GHC] #11560: panic: isInjectiveTyCon sees a TcTyCon
by GHC 27 Jul '17

27 Jul '17

#11560: panic: isInjectiveTyCon sees a TcTyCon -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I typed this into ghci and it panicked. Not sure whether the declaration is really valid anyways, but ghci shouldn't panic on it. {{{ rwbarton@morphism:~/ghc-newest$ ./inplace/bin/ghc-stage2 --interactive GHCi, version 8.1.20160201: http://www.haskell.org/ghc/ :? for help Prelude> :set -XTypeFamilies -XTypeInType Prelude> :m +Data.Kind Prelude Data.Kind> type family T (l :: *) (k :: l) :: * where { T * Int = * } ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20160201 for x86_64-unknown-linux): isInjectiveTyCon sees a TcTyCon T Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11560> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 9

[GHC] #11672: Poor error message
by GHC 27 Jul '17

27 Jul '17

#11672: Poor error message -------------------------------------+------------------------------------- Reporter: adamgundry | Owner: adamgundry Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1-rc2 (Type checker) | Keywords: ErrorMessages | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: #11198 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- [https://mail.haskell.org/pipermail/haskell-cafe/2016-February/123262.html Daniel Díaz recently pointed out] a particularly terrible error message. Here's a reduced example: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} module BadError where import GHC.TypeLits import Data.Proxy f :: Proxy (a :: Symbol) -> Int f _ = f (Proxy :: Proxy (Int -> Bool)) }}} With GHC 8.0 RC2, this leads to the following error: {{{ • Expected kind ‘Proxy ((->) Int Bool)’, but ‘Data.Proxy.Proxy :: Proxy (Int -> Bool)’ has kind ‘Proxy (Int -> Bool)’ • In the first argument of ‘f’, namely ‘(Proxy :: Proxy (Int -> Bool))’ In the expression: f (Proxy :: Proxy (Int -> Bool)) In an equation for ‘f’: f _ = f (Proxy :: Proxy (Int -> Bool)) }}} or with `-fprint-explicit-kinds -fprint-explicit-coercions`: {{{ • Expected kind ‘Proxy Symbol (((->) |> <*>_N -> <*>_N -> U(hole:{aCy}, *, Symbol)_N) Int Bool)’, but ‘(Data.Proxy.Proxy) @ k_aCv @ t_aCw :: Proxy (Int -> Bool)’ has kind ‘Proxy * (Int -> Bool)’ }}} As Iavor, Richard and I discussed, this message has at least three separate problems: * It says `kind` when it should say `type`. * `((->) Int Bool)` is printed instead of `Int -> Bool` (because there is a coercion hiding in the type). * The real error is the insoluble constraint `Symbol ~ *`, which is not reported at all! The first two should be fairly easy to fix. For the third, when reporting insoluble constraints, we should prefer to report those on which no other constraints depend. (In this case, the presence of `hole:{aCy}` in the constraint is an explicit dependency on the other constraint.) I'll try to take a look at this. It is no doubt related to #11198. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11672> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 6