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[GHC] #14831: QuantifiedConstraints: Odd superclass constraint
by GHC 05 Jun '18

05 Jun '18

#14831: QuantifiedConstraints: Odd superclass constraint -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This code works {{{#!hs {-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances #-} import Data.Semigroup newtype Free cls a = Free () instance (forall xx. cls xx => Semigroup xx) => Semigroup (Free cls a) where (<>) = undefined stimes = undefined sconcat = undefined -- instance (forall xx. cls xx => Semigroup xx) => Monoid (Free cls a) where -- mempty = undefined }}} but uncomment the `Monoid` instance and we get {{{ $ ghci -ignore-dot-ghci /tmp/I.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( /tmp/I.hs, interpreted ) /tmp/I.hs:12:10: error: • Could not deduce: cls (Free cls a) arising from the superclasses of an instance declaration from the context: forall xx. cls xx => Semigroup xx bound by the instance declaration at /tmp/I.hs:12:10-67 • In the instance declaration for ‘Monoid (Free cls a)’ | 12 | instance (forall xx. cls xx => Semigroup xx) => Monoid (Free cls a) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> }}} Is this correct behavior? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14831> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4806: Make error message more user friendly when module is not found because package is unusable
by GHC 05 Jun '18

05 Jun '18

#4806: Make error message more user friendly when module is not found because package is unusable -------------------------------------+------------------------------------- Reporter: mitar | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.0.1 Resolution: | Keywords: newcomer Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4783 Wiki Page: | -------------------------------------+------------------------------------- Changes (by sgillespie): * differential: => Phab:D4783 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4806#comment:29> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14877: QuantifiedConstraints: Can't deduce `xx' from `(xx => a, xx)'
by GHC 05 Jun '18

05 Jun '18

#14877: QuantifiedConstraints: Can't deduce `xx' from `(xx => a, xx)' -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints, wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This works with the [https://ghc.haskell.org/trac/ghc/ticket/2893#comment:34 latest change to the -XQuantifiedConstraints branch]. {{{#!hs {-# Language QuantifiedConstraints, FlexibleInstances, MultiParamTypeClasses, AllowAmbiguousTypes, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances #-} class (forall xx. (xx => a) => Implies xx b) => F a b instance (forall xx. (xx => a) => Implies xx b) => F a b class (a => b) => Implies a b instance (a => b) => Implies a b }}} but replacing `Implies xx b` with `(xx => b)` causes it to fail. I don't know if the cause of this overlaps with an existing ticket. {{{#!hs class (forall xx. (xx => a) => (xx => b)) => F a b instance (forall xx. (xx => a) => (xx => b)) => F a b -- $ ghci -ignore-dot-ghci /tmp/ASD.hs -- GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help -- [1 of 1] Compiling Main ( /tmp/ASD.hs, interpreted ) -- -- /tmp/ASD.hs:4:10: error: -- • Could not deduce: xx0 -- arising from the superclasses of an instance declaration -- from the context: forall (xx :: Constraint). (xx => a, xx) => b -- bound by the instance declaration at /tmp/ASD.hs:4:10-53 -- or from: (xx => a, xx) -- bound by a quantified context at /tmp/ASD.hs:1:1 -- • In the instance declaration for ‘F a b’ -- | -- 4 | instance (forall xx. (xx => a) => (xx => b)) => F a b -- | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ -- Failed, no modules loaded. -- Prelude> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14877> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14832: QuantifiedConstraints: Strengthening context causes failure
by GHC 05 Jun '18

05 Jun '18

#14832: QuantifiedConstraints: Strengthening context causes failure -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- With Simon's latest changes (ticket:14733#comment:9) this works: {{{#!hs {-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances, GADTs #-} import Control.Category import Data.Kind import Data.Coerce data With cls a b where With :: cls a b => With cls a b type Coercion = With Coercible type Refl rel = (forall xx. rel xx xx :: Constraint) type Trans rel = (forall xx yy zz. (rel xx yy, rel yy zz) => rel xx zz :: Constraint) instance Refl rel => Category (With rel) where id = With (.) = undefined }}} But strengthening the context with `Trans rel`: {{{#!hs instance (Refl rel, Trans rel) => Category (With rel) where }}} causes it to fail {{{ GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( J.hs, interpreted ) J.hs:15:10: error: • Could not deduce: rel xx zz from the context: (Refl rel, Trans rel) bound by an instance declaration: forall k (rel :: k -> k -> Constraint). (Refl rel, Trans rel) => Category (With rel) at J.hs:15:10-53 or from: (rel xx yy, rel yy zz) bound by a quantified context at J.hs:15:10-53 • In the ambiguity check for an instance declaration To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the instance declaration for ‘Category (With rel)’ | 15 | instance (Refl rel, Trans rel) => Category (With rel) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ J.hs:15:10: error: • Could not deduce: rel xx xx from the context: (Refl rel, Trans rel) bound by an instance declaration: forall k (rel :: k -> k -> Constraint). (Refl rel, Trans rel) => Category (With rel) at J.hs:15:10-53 • In the ambiguity check for an instance declaration To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the instance declaration for ‘Category (With rel)’ | 15 | instance (Refl rel, Trans rel) => Category (With rel) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14832> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8516: Add (->) representation and the Invariant class to GHC.Generics
by GHC 04 Jun '18

04 Jun '18

#8516: Add (->) representation and the Invariant class to GHC.Generics ----------------------------------------------+---------------------------- Reporter: nfrisby | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type checker) | Version: 7.7 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: ----------------------------------------------+---------------------------- We currently disallow any use of the parameter in the domain of (->). {{{ newtype F a = F ((a -> Int) -> Int) deriving Generic1 <interactive>:4:38: Can't make a derived instance of `Generic1 (F g)': Constructor `F' must use the last type parameter only as the last argument of a data type, newtype, or (->) In the data declaration for `F' }}} !DeriveFunctor succeeds for this F. I'd like to add this representation type to GHC.Generics and !DeriveGeneric. {{{ newtype (f :->: g) a = FArrow1 (f a -> g a) }}} We could then represent the first example above. We could also derive the more interesting Generic1 (F g). {{{ newtype F g a = F (g a -> Int) deriving Generic1 type instance Rep1 (F g) = Rec1 g :->: Rec0 Int instance Generic1 (F g) where to x = F $ unRec0 . unArrow1 x . Rec1 from (F x) = FArrow1 $ Rec0 . x . unRec1 }}} Admittedly, there's not many generic definitions impeded by not having (:->:). Contra- and in-variant types are uncommon. I'm suggesting this feature without strong motivating examples because I think this would streamline the implementation of -XDeriveGenerics in some ways while also making it more general — assuming that we added the Invariant class to base or ghc-prim. {{{ class Invariant t where invmap :: (a -> b) -> (b -> a) -> t a -> t b invmap_covariant :: Functor t => (a -> b) -> (b -> a) -> t a -> t b invmap_covariant f _ = fmap f instance (Invariant f,Invariant g) => Invariant (FArrow f g) where invmap co contra (FArrow h) = FArrow $ invmap co contra . h . invmap contra co }}} (Of course, Invariant should be a super class of Functor. :/ ) Now we can handle quite involved examples: {{{ newtype F g h a = F (g (h a)) deriving Generic1 instance Invariant g => Generic1 (F g h) where to x = invmap unRec1 Rec1 $ unComp1 x from (F x) = Comp1 $ invmap Rec1 unRec1 }}} All of that said, I'm mostly opening this ticket so I can get feedback on difficulties I might not be anticipating and have a place to reference from the compiler source code comments. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8516> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #436: Declare large amounts of constant data
by GHC 04 Jun '18

04 Jun '18

#436: Declare large amounts of constant data -------------------------------------+------------------------------------- Reporter: ajk | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: ⊥ Component: Compiler | Version: None Resolution: duplicate | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: #9719 | -------------------------------------+------------------------------------- Changes (by bgamari): * status: new => closed * failure: => None/Unknown * resolution: None => duplicate * wikipage: => #9719 Comment: I think this will be addressed with the solution to #9719 (https://github.com/ghc-proposals/ghc- proposals/pull/135/files?short_path=d675984#diff- d6759846a84df1af033ef6e51346e80f) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/436#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14939: StaticPointers + -dcore-lint: cause Core Lint error??
by GHC 04 Jun '18

04 Jun '18

#14939: StaticPointers + -dcore-lint: cause Core Lint error?? -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple StaticPointers | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This is an odd bug {{{#!hs {-# Language StaticPointers #-} import Data.Kind type Cat ob = ob -> ob -> Type type Alg cls ob = ob newtype Frí (cls::Type -> Constraint) :: (Type -> Alg cls Type) where Frí :: { with :: forall x. cls x => (a -> x) -> x } -> Frí cls a data AlgCat (cls::Type -> Constraint) :: Cat (Alg cls Type) where AlgCat :: (cls a, cls b) => (a -> b) -> AlgCat cls a b leftAdj :: AlgCat cls (Frí cls a) b -> (a -> b) leftAdj (AlgCat f) a = undefined }}} causes a {{{ $ ./ghc-stage2 --interactive -ignore-dot-ghci -dcore-lint 222-bug.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( 222-bug.hs, interpreted ) *** Core Lint errors : in result of Float out(FOS {Lam = Just 0, Consts = True, OverSatApps = False}) *** <no location info>: warning: In the type ‘forall (cls :: * -> Constraint) (b :: Alg cls *). b’ Variable escape in forall: forall (cls :: * -> Constraint) (b :: Alg cls *). b *** Offending Program *** with :: forall (cls :: * -> Constraint) a. Frí cls a -> forall x. cls x => (a -> x) -> x [LclIdX[[RecSel]], Arity=2] with .. . --->8------->8------->8--- .. }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14939> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #2893: Implement "Quantified constraints" proposal
by GHC 04 Jun '18

04 Jun '18

#2893: Implement "Quantified constraints" proposal -------------------------------------+------------------------------------- Reporter: porges | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: 6.10.1 Resolution: | Keywords: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #5927 | Differential Rev(s): Phab:D4724 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari <ben@…>): In [changeset:"7df589608abb178efd6499ee705ba4eebd0cf0d1/ghc" 7df5896/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="7df589608abb178efd6499ee705ba4eebd0cf0d1" Implement QuantifiedConstraints We have wanted quantified constraints for ages and, as I hoped, they proved remarkably simple to implement. All the machinery was already in place. The main ticket is Trac #2893, but also relevant are #5927 #8516 #9123 (especially! higher kinded roles) #14070 #14317 The wiki page is https://ghc.haskell.org/trac/ghc/wiki/QuantifiedConstraints which in turn contains a link to the GHC Proposal where the change is specified. Here is the relevant Note: Note [Quantified constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The -XQuantifiedConstraints extension allows type-class contexts like this: data Rose f x = Rose x (f (Rose f x)) instance (Eq a, forall b. Eq b => Eq (f b)) => Eq (Rose f a) where (Rose x1 rs1) == (Rose x2 rs2) = x1==x2 && rs1 >= rs2 Note the (forall b. Eq b => Eq (f b)) in the instance contexts. This quantified constraint is needed to solve the [W] (Eq (f (Rose f x))) constraint which arises form the (==) definition. Here are the moving parts * Language extension {-# LANGUAGE QuantifiedConstraints #-} and add it to ghc-boot-th:GHC.LanguageExtensions.Type.Extension * A new form of evidence, EvDFun, that is used to discharge such wanted constraints * checkValidType gets some changes to accept forall-constraints only in the right places. * Type.PredTree gets a new constructor ForAllPred, and and classifyPredType analyses a PredType to decompose the new forall-constraints * Define a type TcRnTypes.QCInst, which holds a given quantified constraint in the inert set * TcSMonad.InertCans gets an extra field, inert_insts :: [QCInst], which holds all the Given forall-constraints. In effect, such Given constraints are like local instance decls. * When trying to solve a class constraint, via TcInteract.matchInstEnv, use the InstEnv from inert_insts so that we include the local Given forall-constraints in the lookup. (See TcSMonad.getInstEnvs.) * topReactionsStage calls doTopReactOther for CIrredCan and CTyEqCan, so they can try to react with any given quantified constraints (TcInteract.matchLocalInst) * TcCanonical.canForAll deals with solving a forall-constraint. See Note [Solving a Wanted forall-constraint] Note [Solving a Wanted forall-constraint] * We augment the kick-out code to kick out an inert forall constraint if it can be rewritten by a new type equality; see TcSMonad.kick_out_rewritable Some other related refactoring ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Move SCC on evidence bindings to post-desugaring, which fixed #14735, and is generally nicer anyway because we can use existing CoreSyn free-var functions. (Quantified constraints made the free-vars of an ev-term a bit more complicated.) * In LookupInstResult, replace GenInst with OneInst and NotSure, using the latter for multiple matches and/or one or more unifiers }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2893#comment:39> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #5927: A type-level "implies" constraint on Constraints
by GHC 04 Jun '18

04 Jun '18

#5927: A type-level "implies" constraint on Constraints -------------------------------------+------------------------------------- Reporter: illissius | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 7.4.1 checker) | Keywords: Resolution: duplicate | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #2893 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari <ben@…>): In [changeset:"7df589608abb178efd6499ee705ba4eebd0cf0d1/ghc" 7df5896/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="7df589608abb178efd6499ee705ba4eebd0cf0d1" Implement QuantifiedConstraints We have wanted quantified constraints for ages and, as I hoped, they proved remarkably simple to implement. All the machinery was already in place. The main ticket is Trac #2893, but also relevant are #5927 #8516 #9123 (especially! higher kinded roles) #14070 #14317 The wiki page is https://ghc.haskell.org/trac/ghc/wiki/QuantifiedConstraints which in turn contains a link to the GHC Proposal where the change is specified. Here is the relevant Note: Note [Quantified constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The -XQuantifiedConstraints extension allows type-class contexts like this: data Rose f x = Rose x (f (Rose f x)) instance (Eq a, forall b. Eq b => Eq (f b)) => Eq (Rose f a) where (Rose x1 rs1) == (Rose x2 rs2) = x1==x2 && rs1 >= rs2 Note the (forall b. Eq b => Eq (f b)) in the instance contexts. This quantified constraint is needed to solve the [W] (Eq (f (Rose f x))) constraint which arises form the (==) definition. Here are the moving parts * Language extension {-# LANGUAGE QuantifiedConstraints #-} and add it to ghc-boot-th:GHC.LanguageExtensions.Type.Extension * A new form of evidence, EvDFun, that is used to discharge such wanted constraints * checkValidType gets some changes to accept forall-constraints only in the right places. * Type.PredTree gets a new constructor ForAllPred, and and classifyPredType analyses a PredType to decompose the new forall-constraints * Define a type TcRnTypes.QCInst, which holds a given quantified constraint in the inert set * TcSMonad.InertCans gets an extra field, inert_insts :: [QCInst], which holds all the Given forall-constraints. In effect, such Given constraints are like local instance decls. * When trying to solve a class constraint, via TcInteract.matchInstEnv, use the InstEnv from inert_insts so that we include the local Given forall-constraints in the lookup. (See TcSMonad.getInstEnvs.) * topReactionsStage calls doTopReactOther for CIrredCan and CTyEqCan, so they can try to react with any given quantified constraints (TcInteract.matchLocalInst) * TcCanonical.canForAll deals with solving a forall-constraint. See Note [Solving a Wanted forall-constraint] Note [Solving a Wanted forall-constraint] * We augment the kick-out code to kick out an inert forall constraint if it can be rewritten by a new type equality; see TcSMonad.kick_out_rewritable Some other related refactoring ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Move SCC on evidence bindings to post-desugaring, which fixed #14735, and is generally nicer anyway because we can use existing CoreSyn free-var functions. (Quantified constraints made the free-vars of an ev-term a bit more complicated.) * In LookupInstResult, replace GenInst with OneInst and NotSure, using the latter for multiple matches and/or one or more unifiers }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5927#comment:35> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14070: Allow ‘unsafe’ deriving strategy, deriving code with ‘unsafeCoerce’
by GHC 04 Jun '18

04 Jun '18

#14070: Allow ‘unsafe’ deriving strategy, deriving code with ‘unsafeCoerce’ -------------------------------------+------------------------------------- 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 {{{#!hs newtype M m a = M (m a) deriving newtype Functor }}} produces something similar to {{{#!hs instance Functor m => Functor (M m) where fmap :: forall a a'. (a -> a') -> (M m a -> M m a') fmap = coerce (fmap @m @a @a') }}} but as wiki:Roles2 points out, this fails for methods such as `join_`: {{{#!hs class MonadJoin m where join_ :: m (m a) -> m a newtype M m a = M (m a) deriving newtype MonadJoin -- Couldn't match representation of type ‘m (M m a)’ with that of ‘m (m a)’ }}} I think the user should be given the option to respect the roles or not by supplying `unsafe` wiki:Commentary/Compiler/DerivingStrategies {{{#!hs newtype M m a = M (m a) deriving unsafe newtype MonadJoin -- -( Produces `unsafeCoerce' instead of `coerce' )- -- instance MonadJoin m => MonadJoin (M m) where -- join_ :: forall a. M m (M m a) -> M m a -- join_ = unsafeCoerce (join_ @m @a) }}} This lets us (`newtype`-) derive a host of instances that we currently can not: `Traversable`, `Distributive` (currently forbidden due to [https://hackage.haskell.org/package/distributive-0.5.2/docs/Data- Distributive.html#v:distribute distribute]), [https://hackage.haskell.org/package/linear-1.20.7/docs/Linear- Matrix.html#v:trace Trace], [http://hackage.haskell.org/package/profunctors-5/docs/Data-Profunctor- Monad.html ProfunctorMonad / ProfunctorComonad], ... It does not seem like lenses (`FunctorWithIndex`, `FoldableWithIndex`, `TraversableWithIndex`) work with this though -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14070> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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