wiki:Roles2

Version 1 (modified by goldfire, 9 months ago) (diff)

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Update to Roles

It has become (somewhat) clear that the Roles mechanism as implemented in GHC 7.8 is insufficient. (See examples below.) This page is dedicated to creating a new design for roles that might fix the problems.

Problem examples

We have three known examples of where current roles are failing us.

Adding join to Monad

As part of the Applicative-Monad Proposal, we wish to add join to Monad, thus:

class Applicative m => Monad m where
  ...
  join :: forall a. m (m a) -> m a

This is all well and good, and would work with GeneralizedNewtypeDeriving (GND) most of the time. But, consider this:

newtype T m a = T (m a)
  deriving (Functor, Applicative, Monad)