New Functor => Applicative => Monad Hierarchy

[gidyn]

The standard class hierarchy is a consequence of Haskell's historical development, rather than logic. I would therefore like to propose a reform of the Functor, Applicative, and Monad type classes. The new hierarchy is logical, eliminates many duplicate names from the standard type class definitions, and removes the need for boilerplate Monad -> Applicative instance declarations.

The proposal is detailed in ​the wiki, along with an example of a legacy module to provide some backwards-compatibility.

[gidyn]

Formally proposing this on the mailing lists, with a discussion deadline of 1 February.

[basvandijk]

Prepare GHC for the new monad hierarchy

[basvandijk]

Patch for base that implements the new monad hierarchy

[basvandijk]

Patch for happy that generates a Functor and Applicative instance for HappyIdentity?

[basvandijk]

All my patches can also be pulled from my publicly available ghc repository:

darcs pull http://bifunctor.homelinux.net/~bas/ghc/
darcs pull http://bifunctor.homelinux.net/~bas/ghc/libraries/base
darcs pull http://bifunctor.homelinux.net/~bas/ghc/...

Also see the ​generated haddock documentation of the base library with the patches applied.

[gidyn]

Please note that the attached patches only implement the new Applicative => Monad hierarchy, but do not change any names (as proposed on the wiki page). The deprecation/renaming of redundant definitions may be proposed in a separate ticket.

[igloo]

Proposal tickets are no longer needed as part of the library submissions process. Instead, a normal ticket should be created once consensus has been achieved. Please see the ​process description for details.