#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