#10831: DeriveFunctor not capable of deriving when wrapping Either -------------------------------------+------------------------------------- Reporter: htebalaka | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 7.10.2 checker) | 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: -------------------------------------+------------------------------------- Changes (by thomie): * os: MacOS X => Unknown/Multiple * component: Compiler => Compiler (Type checker) Old description:

GHC can derive Functor for Okay, but rejects the equivalent Bad. {{{#!hs {-# LANGUAGE DeriveFunctor #-} data Okay f g a b = Inl (f a b) | Inr (g a b) deriving (Functor) newtype Bad f g a b = Bad (Either (f a b) (g a b)) deriving (Functor) }}}

New description: GHC can derive Functor for Okay, but rejects the equivalent Bad. {{{#!hs {-# LANGUAGE DeriveFunctor #-} data Okay f g a b = Inl (f a b) | Inr (g a b) deriving (Functor) newtype Bad f g a b = Bad (Either (f a b) (g a b)) deriving (Functor) }}} {{{ Can't make a derived instance of ‘Functor (Bad f g a)’: Constructor ‘Bad’ must use the type variable only as the last argument of a data type In the data declaration for ‘Bad’ }}} -- Comment: This comment (option `c`) in `compiler/typecheck/TcDeriv.hs` says this is to be expected: {{{ cond_functorOK :: Bool -> Bool -> Condition -- OK for Functor/Foldable/Traversable class -- Currently: (a) at least one argument -- (b) don't use argument contravariantly -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a) -- (d) optionally: don't use function types -- (e) no "stupid context" on data type }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10831#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10831: Document conditions on deriving Functor -------------------------------------+------------------------------------- Reporter: htebalaka | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Documentation | Unknown/Multiple bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by simonpj): * cc: RyanGlScott (added) Comment: Ryan Scott made an implementation-oriented stab at documenting Functor, Foldable, Traversable here: [wiki:Commentary/Compiler/DeriveFunctor]. But I think we still lack a user-oriented specification, and such a thing would be tremendously welcome. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10831#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

Replying to [comment:3 simonpj]:

...

But I think we still lack a user-oriented specification, and such a

#10831: Document conditions on deriving Functor -------------------------------------+------------------------------------- Reporter: htebalaka | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Documentation | Unknown/Multiple bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by rwbarton): Replying to [comment:4 RyanGlScott]: thing would be tremendously welcome.

What exactly would we want from a "user-oriented specification"? Do we

want to see the compile-time error messages improved? Or do we want to see particular portions of the Commentary page and/or users' guide fleshed out? The user's guide should contain an explanation sufficient for a user to understand what types will and will not work with deriving Functor. Ideally this should contain examples of both sorts. The wiki is intended for developer-facing documentation; users shouldn't have to consult the wiki to find out how to use a feature. I think the errors (at least the error appearing in this ticket; I haven't looked at others if there are any) are fine, assuming that there is documentation added to the user's guide that explains in more detail what is wrong here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10831#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10831: Document conditions on deriving Functor -------------------------------------+------------------------------------- Reporter: htebalaka | Owner: RyanGlScott Type: bug | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Documentation | Unknown/Multiple bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1293 -------------------------------------+------------------------------------- Changes (by RyanGlScott): * owner: => RyanGlScott * differential: => Phab:D1293 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10831#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10831: Document conditions on deriving Functor -------------------------------------+------------------------------------- Reporter: htebalaka | Owner: RyanGlScott Type: bug | Status: merge Priority: low | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Documentation | Unknown/Multiple bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1293 Wiki Page: | -------------------------------------+------------------------------------- Changes (by thoughtpolice): * status: new => merge * milestone: => 8.0.1 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10831#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler