#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by rwbarton): FWIW this isn't a regression, 7.8 fails with the same error and 7.6 bails out even earlier. And isn't 7.10's behavior in fact the correct one? The instance produced by 7.11 is ill-kinded: {{{ instance [safe] forall (k :: BOX) (f :: k -> *) (g :: * -> k). (Functor f, Functor g) => Functor (Compose f g) }}} `Functor`'s argument must really be of kind `* -> *`. (Though I don't know if this will cause any harm beyond confusing error messages down the line.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by rwbarton): The instance head `Functor (Compose f g)` is well-kinded, but the context `(Functor f, Functor g)` is not unless `k = *`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by rwbarton): Good question. In principle there are classes at other kinds `Functor1`, `Functor2` (not part of a systematic naming scheme) for which you can write {{{ instance (Functor1 (f :: (* -> *) -> *), Functor2 (g :: * -> (* -> *))) => Functor (Compose f g) }}} but GHC will probably never be able to derive that instance. (Maybe if eventually get a polykinded `Functor`, but then the deriving clause could produce a kind-polymorphic instance and there is no problem.) So, I don't see any real issue with having ordinary `deriving` producing an instance for `Functor (Compose * f g)`, to write the kind variable explicitly. However, it's certainly more clear-cut with the standalone deriving declaration, since then the kind variable is determined by the context which you wrote explicitly. I think GHC may have some general principles regarding ordinary deriving declarations and how they are less general than standalone deriving, but I never understood the exact details (aside from the fact that a standalone deriving declaration lets you specify the context). Maybe they don't have anything to say about this case with a kind variable anyways. Not sure where that leaves this ticket; the behavior of HEAD is a bug that I'll file separately. Maybe a feature request dependent on the resolution of that bug. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #10561 | Differential Revisions: -------------------------------------+------------------------------------- Comment (by RyanGlScott): It looks like {{{PolyKinds}}} also breaks deriving {{{Data}}} instances as well: {{{#!hs {-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE PolyKinds #-} import Data.Data newtype WrappedFunctor f a = WrapFunctor (f a) deriving (Data, Typeable) }}} This will result in the error: {{{ No instance for (Typeable a) arising from the 'deriving' clause of a data type declaration Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (Data (WrappedFunctor f a)) }}} (Presumably, it's trying to infer the instance context {{{Data (f a), Typeable f, Typeable a) => Data (WrappedFunctor f a)}}}, but can't.) I'm not sure if this is due to the same underlying issue, but it seems likely, since that code will compile without {{{PolyKinds}}}. I reproduced this on GHC 7.8, 7.10, and HEAD. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #10561 | Differential Revisions: -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"0e1e7987bdcec0e9be309cbe97fa1c92551997f7/ghc"]: {{{ #!CommitTicketReference repository="ghc" revision="0e1e7987bdcec0e9be309cbe97fa1c92551997f7" Test Trac #10524 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #10561 | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): OK, so I fixed comment:7. But the original bug report remains {{{ newtype Compose f g a = Compose (f (g a)) deriving Functor }}} should be fine, but elicits {{{ T10524.hs:5:52: error: Couldn't match kind ‘k’ with ‘*’ arising from the first field of ‘Compose’ (type ‘f (g a)’) When deriving the instance for (Functor (Compose f g)) }}} After all, this explicit instance declaration typechecks fine {{{ instance (Functor f, Functor g) => Functor (Compose f g) where fmap fn (Compose x) = Compose (fmap (fmap fn) x) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #10561 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Richard Eisenberg ): In [changeset:"6746549772c5cc0ac66c0fce562f297f4d4b80a2/ghc" 67465497/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="6746549772c5cc0ac66c0fce562f297f4d4b80a2" Add kind equalities to GHC. This implements the ideas originally put forward in "System FC with Explicit Kind Equality" (ICFP'13). There are several noteworthy changes with this patch: * We now have casts in types. These change the kind of a type. See new constructor `CastTy`. * All types and all constructors can be promoted. This includes GADT constructors. GADT pattern matches take place in type family equations. In Core, types can now be applied to coercions via the `CoercionTy` constructor. * Coercions can now be heterogeneous, relating types of different kinds. A coercion proving `t1 :: k1 ~ t2 :: k2` proves both that `t1` and `t2` are the same and also that `k1` and `k2` are the same. * The `Coercion` type has been significantly enhanced. The documentation in `docs/core-spec/core-spec.pdf` reflects the new reality. * The type of `*` is now `*`. No more `BOX`. * Users can write explicit kind variables in their code, anywhere they can write type variables. For backward compatibility, automatic inference of kind-variable binding is still permitted. * The new extension `TypeInType` turns on the new user-facing features. * Type families and synonyms are now promoted to kinds. This causes trouble with parsing `*`, leading to the somewhat awkward new `HsAppsTy` constructor for `HsType`. This is dispatched with in the renamer, where the kind `*` can be told apart from a type-level multiplication operator. Without `-XTypeInType` the old behavior persists. With `-XTypeInType`, you need to import `Data.Kind` to get `*`, also known as `Type`. * The kind-checking algorithms in TcHsType have been significantly rewritten to allow for enhanced kinds. * The new features are still quite experimental and may be in flux. * TODO: Several open tickets: #11195, #11196, #11197, #11198, #11203. * TODO: Update user manual. Tickets addressed: #9017, #9173, #7961, #10524, #8566, #11142. Updates Haddock submodule. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #10561 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Yes, but test `deriving/should_compile/T10561` still fails: {{{ newtype Compose f g a = Compose (f (g a)) deriving Functor }}} (with `-XPolyKinds`) produces {{{ T10561.hs:10:52: error: • Couldn't match kind ‘k’ with ‘*’ arising from the first field of ‘Compose’ (type ‘f (g a)’) • When deriving the instance for (Functor (Compose f g)) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #10561 | Differential Rev(s): Phab:D2097 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => patch * differential: => Phab:D2097 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10524: PolyKinds doesn't interact well with DeriveFunctor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.1 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | deriving/should_compile/T10561 Blocked By: | Blocking: Related Tickets: #10561 | Differential Rev(s): Phab:D2097 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: patch => closed * testcase: => deriving/should_compile/T10561 * resolution: => fixed * milestone: => 8.0.1 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10524#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler