August 2017 - ghc-tickets

[GHC] #14063: Compiling with --backpack with undefined dependency results in "the 'impossible' happened"
by GHC 01 Aug '17

01 Aug '17

#14063: Compiling with --backpack with undefined dependency results in "the 'impossible' happened" -------------------------------------+------------------------------------- Reporter: rcook | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: Backpack | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Using GHC 8.2.1: {{{ > ghc --version The Glorious Glasgow Haskell Compilation System, version 8.2.1 }}} And compiling with `--backpack` as follows: {{{ > ghc --backpack foo.bkp [1 of 4] Processing foo-indef [1 of 2] Compiling Str[sig] ( foo-indef\Str.hsig, nothing ) [2 of 2] Compiling Foo ( foo-indef\Foo.hs, nothing ) [2 of 4] Processing foo-string Instantiating foo-string [1 of 1] Compiling Str ( foo-string\Str.hs, foo- string\Str.o ) [3 of 4] Processing foo-int Instantiating foo-int [1 of 1] Compiling Str ( foo-int\Str.hs, foo-int\Str.o ) [4 of 4] Processing main ghc.EXE: panic! (the 'impossible' happened) (GHC version 8.2.1 for x86_64-unknown-mingw32): no package name CallStack (from HasCallStack): error, called at compiler\backpack\DriverBkp.hs:573:32 in ghc:DriverBkp Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug make: *** [foo-indef/Foo.hi] Error 1 }}} See attached `foo.bkp` file. While the Backpack file ''is'' invalid, in that the `main` unit mentions the `foo` dependency, which does not exist, this shouldn't lead to a GHC panic. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14063> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14058: Cannot bundle pattern synonym with exported data family
by GHC 01 Aug '17

01 Aug '17

#14058: Cannot bundle pattern synonym with exported data family -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple PatternSynonyms | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I'd like to write this code: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE TypeOperators #-} module Foo (Sing(.., SCons)) where data family Sing (a :: k) data instance Sing (z :: [a]) where SNil :: Sing '[] (:%) :: Sing x -> Sing xs -> Sing (x:xs) pattern SCons :: forall a (z :: [a]). () => forall (x :: a) (xs :: [a]). z ~ (x:xs) => Sing x -> Sing xs -> Sing z pattern SCons x xs = (:%) x xs {-# COMPLETE SNil, SCons #-} }}} But alas, GHC will not let me: {{{ $ ghci Bug.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Foo ( Bug.hs, interpreted ) Bug.hs:7:13: error: • Pattern synonyms can be bundled only with datatypes. • In the pattern synonym: SCons In the export: Sing(.., SCons) | 7 | module Foo (Sing(.., SCons)) where | ^^^^^^^^^^^^^^^ Failed, 0 modules loaded. }}} Can this restriction be lifted for data families? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14058> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13731: DeriveFunctor and friends don't understand type families
by GHC 01 Aug '17

01 Aug '17

#13731: DeriveFunctor and friends don't understand type families -------------------------------------+------------------------------------- Reporter: spacekitteh | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 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 {-# LANGUAGE DeriveFunctor #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE TypeFamilies #-} data Test ext a where Foo :: a -> Test ext a Extend :: (ExtensionType ext a) -> Test ext a type family ExtensionType ext a data ListExtension type instance ExtensionType ListExtension a = [a] deriving instance Functor (Test ListExtension) {- a.hs:15:1: error: • Can't make a derived instance of ‘Functor (Test ListExtension)’: Constructor ‘Extend’ must use the type variable only as the last argument of a data type • In the stand-alone deriving instance for ‘Functor (Test ListExtension)’ | 15 | deriving instance Functor (Test ListExtension) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, modules loaded: none. -} }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13731> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13403: Derive instances (Applicative, Monad, ...) for structures lifted over functors
by GHC 01 Aug '17

01 Aug '17

#13403: Derive instances (Applicative, Monad, ...) for structures lifted over functors -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) 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: -------------------------------------+------------------------------------- I'll start small: Given that we know how to define various instances for [https://hackage.haskell.org/package/base-4.9.1.0/docs/Data-Functor- Product.html Product] GHC could do it automatically. {{{#!hs data P f g a = f a ::: g a deriving (Functor, Applicative, Alternative) {- instance (Applicative f, Applicative g) => Applicative (P f g) where pure x = pure x ::: pure x (f:::g) <*> (x:::y) = (f <*> x) ::: (g <*> y) -} }}} And for specific constructors as well {{{#!hs data Q a = [a] :*: Maybe a deriving (Functor, Applicative, Alternative) {- instance Applicative Q where pure x = [x] :*: Just x (f:*:g) <*> (x:*:y) = (f <*> x) :*: (g <*> y) -} }}} == Alternative == Use `GeneralizedNewtypeDeriving` {{{#!hs newtype Q a = Q (Product [] Maybe a) deriving (Functor, Applicative, Alternative) pattern (:*:) :: [a] -> Maybe a -> Q a pattern a :*: b = Q (Pair a b) }}} == Future Work == This should work for a combination of various things, using `Const _` deprives us of `Alternative` {{{#!hs newtype U e a = U (([] `Product` Maybe `Product` Const e) a) deriving (Functor, Applicative) }}} using sums where [https://hackage.haskell.org/package/transformers-0.5.4.0/docs/Control- Applicative-Lift.html one summand is identity] gives us `Applicative` / `Alternative` {{{#!hs -- data Lift f a = Pure a | Other (f a) import Control.Applicative.Lift data V a = V ((Lift [] `Product` Maybe) a) deriving (Functor, Applicative, Alternative) }}} I want to be able to write this directly {{{#!hs data U e a = U [a] (Maybe a) (Const e a) deriving (Functor, Applicative) data V a = VL a (Maybe a) | VR [a] (Maybe a) deriving (Functor, Applicative, Alternative) }}} == Future, Future Work == [https://hackage.haskell.org/package/kan-extensions-5.0.1/docs/Data- Functor-Kan-Lan.html left-Kan extension] {{{#!hs data Lan g h a where Lan :: (g b -> a) -> h b -> Lan g h a deriving (Functor, Applicative) }}} [https://hackage.haskell.org/package/kan-extensions-5.0.1/docs/Control- Monad-Codensity.html codensity] {{{#!hs data Endo a = Endo (a -> a) newtype CodEndo a = CE (forall xx. (a -> Endo xx) -> Endo xx) deriving (Functor, Applicative, Monad) }}} and [https://hackage.haskell.org/package/free-4.12.4/docs/Control-Comonad- Cofree.html comonad] {{{#!hs data Rose a = a :< [Rose a] deriving (Functor, Applicative, Monad, Comonad, ...) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13403> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9522: SPECIALISE pragmas for derived instances
by GHC 01 Aug '17

01 Aug '17

#9522: SPECIALISE pragmas for derived instances -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- In package `ghc-prim`, in `GHC.Classes` we have {{{ instance (Eq a) => Eq [a] where {-# SPECIALISE instance Eq [[Char]] #-} {-# SPECIALISE instance Eq [Char] #-} {-# SPECIALISE instance Eq [Int] #-} [] == [] = True (x:xs) == (y:ys) = x == y && xs == ys _xs == _ys = False }}} The `SPECIALISE instance` pragmas instantiate the code for these commonly- used types. But for tuples we use `deriving`: {{{ deriving instance (Eq a, Eq b) => Eq (a, b) }}} and many more similar. There is no way to add a `SPECIALISE instance` pragma for a derived instance. This is bad, because they are heavily used. It should probably be possible to have a top-level {{{ {-# SPECIALISE instance Eq (Int, Bool) #-} }}} even in another module (as we can now do for functions. To do this right, we'd need to make the code for derived methods `INLINEALBE`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9522> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8827: Inferring Safe mode with GeneralizedNewtypeDeriving is wrong
by GHC 01 Aug '17

01 Aug '17

#8827: Inferring Safe mode with GeneralizedNewtypeDeriving is wrong ------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.1 Component: Compiler | Version: 7.8.1-rc1 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- Consider the following modules: {{{ module A (List, ints) where data List a = Nil | Cons a (List a) infixr 4 `Cons` ints :: List Int ints = 1 `Cons` 2 `Cons` 3 `Cons` Nil }}} {{{ {-# LANGUAGE GeneralizedNewtypeDeriving #-} module B where import A newtype Age = MkAge Int deriving C class C a where cList :: List a instance C Int where cList = ints }}} {{{ {-# LANGUAGE Safe #-} module C (ages) where import A import B ages :: List Age ages = cList }}} Module C compiles without a hiccup. But, it shouldn't: the coercion between `ages` and `ints` (performed by !GeneralizedNewtypeDeriving in module B) isn't Safe, as it breaks through `List`'s abstraction. (Note that the constructors of `List` are ''not'' exported from module A!) If module B includes `{-# LANGUAGE Safe #-}`, it duly doesn't compile, because of the stringent "constructors-must-be-in-scope" check done by the `Coercible` mechanism. The problem is that safety can be ''inferred'' without this check taking place. You may also want to read the commentary on #8745 for related discussion. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8827> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13957: Allow deriving multiparameter type classes with representationally equal arguments
by GHC 01 Aug '17

01 Aug '17

#13957: Allow deriving multiparameter type classes with representationally equal arguments -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature | Status: new request | 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: -------------------------------------+------------------------------------- This currently works {{{#!hs {-# Language DerivingStrategies, MultiParamTypeClasses, FunctionalDependencies, GeneralizedNewtypeDeriving #-} import Data.Kind import Data.Functor.Identity class SIEVE f p | p -> f where sIEVE :: p a b -> a -> f b instance SIEVE Identity (->) where sIEVE = (Identity .) newtype ARR a b = ARR (a -> b) deriving newtype (SIEVE Identity) }}} But what if I want a `Sieve I ARR` instance, for `newtype I a = I a` (which is representationally equal to `newtype Identity a = Identity a`)? {{{#!hs {-# Language DerivingStrategies, MultiParamTypeClasses, FunctionalDependencies, GeneralizedNewtypeDeriving, RankNTypes, TypeApplications, ScopedTypeVariables, InstanceSigs #-} import Data.Kind import Data.Functor.Identity import Data.Coerce class SIEVE f p | p -> f where sIEVE :: p a b -> a -> f b instance SIEVE Identity (->) where sIEVE = (Identity .) newtype ARR a b = ARR (a -> b) deriving newtype SIEVE I newtype I a = I a }}} generating the following code (this is basically to code generated before, except replacing some `Identity`s with `I`s {{{#!hs instance SIEVE I ARR where sIEVE :: forall a b. ARR a b -> (a -> I b) sIEVE = coerce @((a -> b) -> a -> Identity b) @(ARR a b -> a -> I b) sIEVE }}} GHC should be able to recover `Identity` due to the functional dependency. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13957> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #5041: Incorrect Read deriving for MagicHash constructors
by GHC 01 Aug '17

01 Aug '17

#5041: Incorrect Read deriving for MagicHash constructors -------------------------------------+------------------------------------- Reporter: dolio | Owner: (none) Type: bug | Status: new Priority: low | Milestone: ⊥ Component: Compiler | Version: 7.0.2 Resolution: | Keywords: deriving Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | deriving/should_run/T5041 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: => deriving -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5041#comment:10> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4815: Instance constraints should be used when deriving on associated data types
by GHC 01 Aug '17

01 Aug '17

#4815: Instance constraints should be used when deriving on associated data types -------------------------------------+------------------------------------- Reporter: batterseapower | Owner: (none) Type: feature request | Status: new Priority: low | Milestone: Component: Compiler | Version: 6.12.3 Resolution: | Keywords: deriving Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #12863 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: => deriving -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4815#comment:20> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #3205: Generalized isomorphic deriving
by GHC 01 Aug '17

01 Aug '17

#3205: Generalized isomorphic deriving -------------------------------------+------------------------------------- Reporter: ajd | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: 6.10.2 Resolution: | Keywords: language, | deriving 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 RyanGlScott): * keywords: language => language, deriving -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3205#comment:4> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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