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[GHC] #13272: DeriveAnyClass regression involving a rigid type variable
by GHC 01 Aug '17

01 Aug '17

#13272: DeriveAnyClass regression involving a rigid type variable -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler | Version: 8.1 (Type checker) | Keywords: Generics | 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: -------------------------------------+------------------------------------- I was writing up an example to show off how `DeriveAnyClass` has improved since 639e702b6129f501c539b158b982ed8489e3d09c, and wouldn't you know it, the example doesn't actually compile anymore post- 639e702b6129f501c539b158b982ed8489e3d09c. Oopsie. {{{#!hs module TypeName where import GHC.Generics class TypeName a where typeName :: forall proxy. proxy a -> String default typeName :: forall proxy d f. (Generic a, Rep a ~ D1 d f, Datatype d) => proxy a -> String typeName _ = gtypeName $ from (undefined :: a) gtypeName :: Datatype d => D1 d f p -> String gtypeName = datatypeName data T a = MkT a deriving (Generic, TypeName) }}} This compiles before that commit. After it, however, it fails with the error: {{{ [1 of 1] Compiling TypeName ( Bug.hs, interpreted ) Bug.hs:23:22: error: • Couldn't match type ‘f’ with ‘C1 ('MetaCons "MkT" 'PrefixI 'False) (S1 ('MetaSel 'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a))’ arising from the 'deriving' clause of a data type declaration ‘f’ is a rigid type variable bound by the deriving clause for ‘TypeName (T a)’ at Bug.hs:14:38 • When deriving the instance for (TypeName (T a)) | 23 | deriving (Generic, TypeName) | ^^^^^^^^ }}} I'm not sure why it's complaining only about `f` and not, say, `d`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13272> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12814: Should GND infer an instance context when deriving method-free classes?
by GHC 01 Aug '17

01 Aug '17

#12814: Should GND infer an instance context when deriving method-free classes? -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new 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: #11369, #12810 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This is a design question that emerged from code that I originally discovered [https://ghc.haskell.org/trac/ghc/ticket/11369#comment:17 here] and [https://ghc.haskell.org/trac/ghc/ticket/12810#comment:3 here]. To recap, it's now possible to have code like this: {{{#!hs {-# LANGUAGE GeneralizedNewtypeDeriving, TypeFamilies #-} class C a where type T a newtype Identity a = Identity a deriving C }}} Compiling this (with `-Wredundant-constraints`) generates this code: {{{#!hs instance C a => C (Identity a) where type T (Identity a) = T a }}} But now GHC will complain: {{{ • Redundant constraint: C a • In the instance declaration for ‘C (Identity a)’ }}} This warning makes sense from the perspective that the `C a` constraint isn't ever used by the associated type family instance. So the question arises: should GND avoid generating an instance context for the representation type in the event it's deriving a class with no methods? Fortunately, patching GHC to do this is trivial: {{{#!diff diff --git a/compiler/typecheck/TcDeriv.hs b/compiler/typecheck/TcDeriv.hs index 4722f16..df2d3d5 100644 --- a/compiler/typecheck/TcDeriv.hs +++ b/compiler/typecheck/TcDeriv.hs @@ -1272,7 +1272,8 @@ mkNewTypeEqn dflags overlap_mode tvs [ let (Pair t1 t2) = mkCoerceClassMethEqn cls dfun_tvs inst_tys rep_inst_ty m in mkPredOrigin (DerivOriginCoerce meth t1 t2) TypeLevel (mkReprPrimEqPred t1 t2) - | meth <- classMethods cls ] + | meth <- meths ] + meths = classMethods cls -- If there are no tyvars, there's no need -- to abstract over the dictionaries we need @@ -1281,7 +1282,11 @@ mkNewTypeEqn dflags overlap_mode tvs -- instance C T -- rather than -- instance C Int => C T - all_preds = rep_pred_o : coercible_constraints ++ sc_theta -- NB: rep_pred comes + all_preds = if null meths then [] else [rep_pred_o] + -- See Note [GND and method-free classes] + ++ coercible_constraints + ++ sc_theta + -- NB: rep_pred_o comes first ------------------------------------------------------------------- -- Figuring out whether we can only do this newtype-deriving thing }}} After implementing this patch, I ran the testsuite, and there were some surprising results. One thing that shocked me was that the program reported in #6088, which had previously failed due to a patch resulting from #8984, was now passing! {{{#!hs {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE EmptyDataDecls #-} module T6088 where class C a newtype A n = A Int type family Pos n data True instance (Pos n ~ True) => C (A n) newtype B n = B (A n) deriving (C) }}} That is because previously, GHC was trying to generate an instance like this: {{{#!hs instance (Pos n ~ True) => C (B n) }}} And this was rejected, since we don't infer exotic equality constraints when deriving. But with this patch, it now generates: {{{#!hs instance {- Empty context => -} C (B n) }}} Which is certainly valid. But is it what a user would expect? I'm not so sure. As hvr notes in #11369, sometimes empty classes are used to enforce invariants, like in the following case: {{{#!hs class Throws e readFoo :: Throws IOError => FilePath -> IO Foo readFoo fn = {- ... -} }}} What if you wanted to have a `Throws` instance for a newtype? You'd probably want something like this: {{{#!hs newtype Id a = MkId a instance Throws a => Throws (Id a) }}} Which feels like something GND should be able to take care of with ease. But to your surprise, you try this: {{{#!hs newtype Id a = MkId a deriving Throws }}} And now GHC generates not the instance above, but rather just: {{{#!hs instance Throws (Identity a) }}} So it's possible that we would lose some of the expressiveness of GND by implementing this change. Is that acceptable? I'm not sure, so I though I'd solicit feedback here. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12814> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13314: StandaloneDeriving and DeriveAnyClass don't work together
by GHC 01 Aug '17

01 Aug '17

#13314: StandaloneDeriving and DeriveAnyClass don't work together -------------------------------------+------------------------------------- Reporter: chris-martin | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 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: -------------------------------------+------------------------------------- I'm starting with the `SPretty` example from the `DeriveAnyClass` section in the GHC users guide: {{{#!hs #!/usr/bin/env stack -- stack --resolver lts-8.0 {-# LANGUAGE DefaultSignatures, DeriveAnyClass, StandaloneDeriving #-} import Prelude import Numeric.Natural (Natural) class SPretty a where sPpr :: a -> String default sPpr :: Show a => a -> String sPpr = show }}} I can write an empty instance for `Natural`: {{{#!hs instance SPretty Natural where }}} So then I would expect to be able to write an equivalent definition using standalone deriving: {{{#!hs deriving instance SPretty Natural }}} But instead it fails with this error: {{{ error: • Can't make a derived instance of ‘SPretty Natural’: The data constructors of ‘Natural’ are not all in scope so you cannot derive an instance for it • In the stand-alone deriving instance for ‘SPretty Natural’ }}} It seems like this ought to work; I'm not sure why the constructors should need to be in scope, given that the instance can be derived trivially without defining any methods. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13314> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4530: Deriving Data for existentially quantified types
by GHC 01 Aug '17

01 Aug '17

#4530: Deriving Data for existentially quantified types -------------------------------------+------------------------------------- Reporter: mitar | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 7.4.1 Component: Compiler | Version: 7.1 Resolution: invalid | Keywords: 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: => deriving -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4530#comment:4> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4529: Deriving Data does not work for attached code
by GHC 01 Aug '17

01 Aug '17

#4529: Deriving Data does not work for attached code -------------------------------------+------------------------------------- Reporter: mitar | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 7.1 checker) | Resolution: invalid | Keywords: 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: => deriving -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4529#comment:4> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4309: Painfully large errors with silly GADT instances
by GHC 01 Aug '17

01 Aug '17

#4309: Painfully large errors with silly GADT instances -------------------------------------+------------------------------------- Reporter: pumpkin | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 6.12.3 Resolution: fixed | Keywords: deriving Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | deriving/should_fail/drvfail011 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: => deriving -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4309#comment:4> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4028: Derived Data instance requires Data instances for unused type parameters
by GHC 01 Aug '17

01 Aug '17

#4028: Derived Data instance requires Data instances for unused type parameters -------------------------------------+------------------------------------- Reporter: igloo | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Not GHC Component: Compiler (Type | Version: 6.12.2 checker) | Resolution: wontfix | Keywords: 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: => deriving -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4028#comment:8> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4019: deriving Ord can produce incorrect and inefficient instances
by GHC 01 Aug '17

01 Aug '17

#4019: deriving Ord can produce incorrect and inefficient instances -------------------------------------+------------------------------------- Reporter: rl | Owner: (none) Type: bug | Status: closed Priority: low | Milestone: Component: Compiler | Version: 6.13 Resolution: fixed | Keywords: deriving 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: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: => deriving -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4019#comment:16> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #3368: Deriving Foldable doesn't work
by GHC 01 Aug '17

01 Aug '17

#3368: Deriving Foldable doesn't work -------------------------------------+------------------------------------- Reporter: cmcq | Owner: igloo Type: bug | Status: closed Priority: normal | Milestone: 6.12.1 Component: Compiler | Version: 6.11 Resolution: fixed | Keywords: 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: => deriving * failure: => None/Unknown -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3368#comment:3> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4896: Deriving Data does not work for attached code
by GHC 01 Aug '17

01 Aug '17

#4896: Deriving Data does not work for attached code -------------------------------------+------------------------------------- Reporter: mitar | Owner: (none) Type: bug | Status: closed Priority: low | Milestone: 7.10.1 Component: Compiler | Version: 7.1 Resolution: fixed | Keywords: deriving Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | deriving/should_compile/T4896 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: => deriving -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4896#comment:14> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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