#13731: DeriveFunctor and friends don't understand type families -------------------------------------+------------------------------------- Reporter: spacekitteh | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | 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: | -------------------------------------+------------------------------------- Comment (by simonpj): The error message is terrible. (Ryan might you look at that?) But the program IS wrong. In Haskell you can't write {{{ f (xs ++ ys) = ... }}} which pattern-matches on a function call. And similarly at the type level you can't pattern match on a function call, as in `instance Functor (Test ListExtension)`. Instead write `instance Functor []`. Oh! We have that instance already; so you can just omit it! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13731#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13731: DeriveFunctor and friends don't understand type families -------------------------------------+------------------------------------- Reporter: spacekitteh | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | 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: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): That instance becomes writable if we mark `ExtensionType` injective {{{#!hs data Test :: Type -> Type -> Type where Foo :: a -> Test ext a Extend :: ExtensionType ext a -> Test ext a type family ExtensionType a = (res :: Type -> Type) | res -> a data ListExtension type instance ExtensionType ListExtension = [] instance Functor (ExtensionType ext) => Functor (Test ext) where fmap :: (a -> a') -> (Test ext a -> Test ext a') fmap f = \case Foo a -> Foo (f a) Extend ex -> Extend (fmap f ex) }}} We need to bump the arity of `ExtensionType` down to one, given that we don't have #10832. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13731#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13731: DeriveFunctor and friends don't understand type families -------------------------------------+------------------------------------- Reporter: spacekitteh | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | 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: | -------------------------------------+------------------------------------- Comment (by RyanGlScott):

So it's not that it must be a data type; it can be a saturated type- family application.

Am I missing something? In the original example, the type family instance is: {{{#!hs type instance ExtensionType ListExtension a = [a] }}} And the instance we're deriving is: {{{#!hs deriving instance Functor (Test ListExtension) }}} In other words, the field of `Extend` will have type `ExtensionType ListExtension a`. This is a fully saturated type family application, right? So surely the rule you've proposed doesn't quite capture the essence of this problem. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13731#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13731: DeriveFunctor and friends don't understand type families -------------------------------------+------------------------------------- Reporter: spacekitteh | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | 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: | -------------------------------------+------------------------------------- Comment (by simonpj): Consider the instance we want: {{{ instance Functor (Test ListExtension) where fmap f (Extend x) = Extend (fmap f x) }}}

Am I missing something?

From the inner `fmap` we get `[W] Functor (ExtensionType ListExtension)`. If `ExtensionType` has arity 2, that would be an un-saturated type family. But if it has arity 1 it is saturated, and reduces to `[]`, so all is well.

... the field of `Extend` will have type `ExtensionType ListExtension a`. This is a fully saturated type family application

Yes, it's saturated in the field, but the use of `fmap` requires us to decompose the type application, we it must be decomposable. It's only decomposable if `ExtensionType` has arity 1. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13731#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13731: DeriveFunctor and friends don't understand type families -------------------------------------+------------------------------------- Reporter: spacekitteh | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | 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: | -------------------------------------+------------------------------------- Comment (by simonpj): You should ask users not me! But looking at {{{ Extend :: ExtensionType ext a -> Test ext a }}} a user might say that `ExtentionType ext a` ''does'' use `a` as the last arg of a saturated type-family application, namely `ExtensionType ext a`! An what is "the last type variable"? Guessing wildly, what about {{{ Illegal use of type variable 'a' in the first argument of `Extend`. Such uses must be of form `ty a` where `ty` is a data type or saturated type family application. }}} I'm not sure if that's better... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13731#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler