[GHC] #16194: deriving, wrong code: newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving stock Functor
#16194: deriving, wrong code: newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving stock Functor -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.7 Keywords: DeriveFunctor | Operating System: Unknown/Multiple deriving RankNTypes | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This fails (`deriving stock Functor`) (data type comes from [https://duplode.github.io/extras/applicative-archery-manuscript.pdf Applicative Archery]) {{{ $ ./ghc-stage2 --interactive -ignore-dot-ghci GHCi, version 8.7.20181230: https://www.haskell.org/ghc/ :? for help Prelude> :set prompt "> "
:set -XRankNTypes -XDeriveFunctor -ddump-deriv :set -dsuppress-idinfo -dsuppress-coercions -dsuppress-type-applications -dsuppress-uniques -dsuppress-module-prefixes newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving Functor
==================== Derived instances ==================== Derived class instances: instance Functor (T cat) where fmap f (MkT a1) = MkT ((\ b2 b3 -> f (b2 ((\ b1 -> b1) b3))) a1) (<$) z (MkT a1) = MkT ((\ b3 b4 -> (\ b2 -> z) (b3 ((\ b1 -> b1) b4))) a1) Derived type family instances: <interactive>:4:62: error: • Couldn't match type ‘forall xx. cat xx xx’ with ‘cat xx0 xx0’ Expected type: cat xx0 xx0 -> a Actual type: (forall xx. cat xx xx) -> a • In the first argument of ‘\ b2 b3 -> f (b2 ((\ b1 -> b1) b3))’, namely ‘a1’ In the first argument of ‘MkT’, namely ‘((\ b2 b3 -> f (b2 ((\ b1 -> b1) b3))) a1)’ In the expression: MkT ((\ b2 b3 -> f (b2 ((\ b1 -> b1) b3))) a1) When typechecking the code for ‘fmap’ in a derived instance for ‘Functor (T cat)’: To see the code I am typechecking, use -ddump-deriv • Relevant bindings include a1 :: (forall xx. cat xx xx) -> a (bound at <interactive>:4:62) fmap :: (a -> b) -> T cat a -> T cat b (bound at <interactive>:4:62) <interactive>:4:62: error: • Couldn't match type ‘forall xx. cat xx xx’ with ‘cat xx1 xx1’ Expected type: cat xx1 xx1 -> b Actual type: (forall xx. cat xx xx) -> b • In the first argument of ‘\ b3 b4 -> (\ b2 -> z) (b3 ((\ b1 -> b1) b4))’, namely ‘a1’ In the first argument of ‘MkT’, namely ‘((\ b3 b4 -> (\ b2 -> z) (b3 ((\ b1 -> b1) b4))) a1)’ In the expression: MkT ((\ b3 b4 -> (\ b2 -> z) (b3 ((\ b1 -> b1) b4))) a1) When typechecking the code for ‘<$’ in a derived instance for ‘Functor (T cat)’: To see the code I am typechecking, use -ddump-deriv • Relevant bindings include a1 :: (forall xx. cat xx xx) -> b (bound at <interactive>:4:62) (<$) :: a -> T cat b -> T cat a (bound at <interactive>:4:62)
}}} Generates odd code {{{#!hs fmap .. = MkT ((\ b2 b3 -> f (b2 ((\ b1 -> b1) b3))) a1) }}} It works fine (in HEAD) to `deriving newtype Functor` (it didn't in 8.2 where I checked) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16194 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16194: deriving, wrong code: newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving stock Functor -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.7 Resolution: | Keywords: DeriveFunctor | deriving RankNTypes Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Fortunately `-XDerivingVia` handles it {{{
newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving Functor via ((->) (forall xx. cat xx xx))
==================== Derived instances ==================== Derived class instances: instance Functor (T cat) where fmap = coerce @((a -> b) -> ((forall (xx :: TYPE LiftedRep). cat xx xx) -> a) -> (forall (xx :: TYPE LiftedRep). cat xx xx) -> b) @((a -> b) -> T cat a -> T cat b) (fmap @((->) (forall (xx :: TYPE LiftedRep). cat xx xx) :: TYPE LiftedRep -> TYPE LiftedRep)) :: forall (a :: TYPE LiftedRep) (b :: TYPE LiftedRep). (a -> b) -> T cat a -> T cat b (<$) = coerce @(a -> ((forall (xx :: TYPE LiftedRep). cat xx xx) -> b) -> (forall (xx :: TYPE LiftedRep). cat xx xx) -> a) @(a -> T cat b -> T cat a) ((<$) @((->) (forall (xx :: TYPE LiftedRep). cat xx xx) :: TYPE LiftedRep -> TYPE LiftedRep)) :: forall (a :: TYPE LiftedRep) (b :: TYPE LiftedRep). a -> T cat b -> T cat a Derived type family instances: }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16194#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16194: deriving, wrong code: newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving stock Functor -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.7 Resolution: | Keywords: DeriveFunctor | deriving RankNTypes Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): This is rather delicate. Here is the instance you likely have in your head: {{{#!hs instance Functor (T cat) where fmap f (MkT g) = MkT (\x -> f (g x)) }}} As `-ddump-deriv` reveals, this isn't quite the code that GHC is producing. It's actually producing this: {{{#!hs instance Functor (T cat) where fmap f (MkT g) = MkT ((\g' x -> f (g' x)) g) }}} Normally, these two implementations would be equivalent. However, because `g` has a higher-rank type `forall xx. cat xx xx`, applying `(\g' x -> f (g' x))` to `g` causes its type to be instantiated with a skolem `xx0`. This is why you get this error message: {{{ Couldn't match type ‘forall xx. cat xx xx’ with ‘cat xx0 xx0’ }}} There are advantages to generating code similar to the second instance. In more complicated examples (e.g., `newtype T a = MkT (Int -> Int -> a)`), using the unapplied form allows for a more compositional code-generation strategy that doesn't have to reason about substituting in arguments. In short, could this be fixed? Theoretically speaking, yes, but it would require a lot of work for not much gain. Personally, I'm inclined to just require users to write out this particular instance by hand. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16194#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16194: deriving, wrong code: newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving stock Functor -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.7 Resolution: | Keywords: DeriveFunctor | deriving RankNTypes Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): That's fine, `newtype` works and it's a contrived example. I agree the gain isn't worth it -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16194#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16194: deriving, wrong code: newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving stock Functor -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.7 Resolution: wontfix | Keywords: DeriveFunctor | deriving RankNTypes 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): * status: new => closed * resolution: => wontfix -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16194#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#16194: deriving, wrong code: newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving stock Functor -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.7 Resolution: wontfix | Keywords: DeriveFunctor | deriving RankNTypes Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Thought: The second representation (`g & \g' -> f (g' x)`) can maybe be captured as a newtype with an instance (like `DList` changes the representation of change the representation of `[a]` to `Endo [a]`). GHC would generate the newtype form, the instance takes care of the operation and coerce back. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16194#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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