#11135: Migrate more of Data.Functor.* from transformers to base. -------------------------------------+------------------------------------- Reporter: hvr | Owner: Type: task | Status: upstream Priority: highest | Milestone: 8.0.1 Component: libraries/base | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1543 Wiki Page: | Phab:D1544 Phab:D1630 -------------------------------------+------------------------------------- Comment (by ekmett): @duairc: The benefit of having `Eq1`, `Ord1`, etc. in the first place is that the resulting instances are fully Haskell 98 and do not require language extensions at all. {{{#!hs instance Eq (f (g a)) => Eq (Compose f g a) }}} requires `FlexibleContexts`. This means that `transformers` which works on *every* compiler since Haskell 98 _without any CPP_ simply could not accept instances of this form. Even allowing for that, it is beneficial to allow things like {{{#!hs instance (Eq1 f, Eq1 g) => Eq1 (Compose f g) }}} because instances like that are more useful in the presence of polymorphic recursion than the at-first-glance simpler pointwise `Eq` instance, because it says something fundamental about how the instances are defined: that it isn't able to do something hinky with the argument type, and do something fundamentally different for `Compose Foo [] Int` vs. `Compose Foo [] Double`, that both have to use the `Eq` or `Ord` or `Read` or `Show` instance for 'Int' or `Double` in a homogeneous way, not separate `FlexibleInstances`. This is confidence you do not get from the pointwise definitions. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11135#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler