Version 2 (modified by goldfire, 18 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) readRef :: MonadIO m => IORef a -> T m a readRef = T . liftIO . readIORef

The designer of `T` will export it abstractly, allowing only the reading of `IORef`s in the `T` monad but not other operations.

Sadly, the use of GND fails here, with `join` in the `Monad` class. Here is the relevant generated code:

instance Monad m => Monad (T m) where ... join = coerce (join :: m (m a) -> m a) :: forall a. T m (T m a) -> T m a

Thus, GHC must derive `Coercible (m (m a) -> m a) (T m (T m a) -> T m a)`. This requirement reduces (in part) to `Coercible (m (m a)) (T m (T m a))`. We try to solve this by applying a newtype-unwrapping instance, `Coercible x (m a) => Coercible x (T m a)`. Then, we must solve `Coercible (m (m a)) (m (T m a))`. And here, we are stuck. We do know `Coercible (m a) (T m a)`, but we don't know the role of `m`'s parameter, and must assume (for safety) that it could be nominal. Thus, we can't solve for `Coercible (m (m a)) (m (T m a))`.

This problem would occur if `join` were in `Monad` and a programmer used GND on any monad transformer. This is a common enough idiom to make us want to fix the situation.

### The `MVector` class

A redesign of the `vector` package is underway, introducing the `Vector` class, among other changes. Here is the offending method:

class (...) => Vector v a where basicUnsafeIndexM :: Monad m => v a -> Int -> m a ...

Here, `a` is the type of the thing stored in the vector, and it is natural to want to coerce a vector of `Int`s to a vector of `Age`s. But, GND would not work here, for very similar reasons to the case above -- we won't be able to coerce `m Int` to `m Age`, because we don't know enough about `m`.

### The `acme-schoenfinkel` package

This next example is the one known case of type-safe code that existed before GHC 7.8 that does not work with GHC 7.8's roles. The package `acme-schoenfinkel-0.1.1` package (by Ertugrul Söylemez) defines

WrappedSchoenfinkel { unwrapSchoenfinkel :: cat a b } deriving (Alternative, Applicative, Arrow, ArrowApply, ArrowChoice, ArrowLoop, ArrowPlus, ArrowZero, Category, Functor)

GND fails on `ArrowApply`:

class Arrow a => ArrowApply (a :: * -> * -> *) where app :: forall b c. a (a b c, b) c

The problem here echoes the `join` problem quite closely.

## The best known solution

Edward Kmett initially described an approach in an email, roughly as follows (names subject to bikeshedding, as always):

Currently, `Data.Type.Coercion` makes this definition:

data Coercion a b where Coercion :: Coercible a b => Coercion a b

Building on that, define a class `Rep` like this:

class Rep (f :: k1 -> k2) where co :: Coercible a b => Coercion (f a) (f b)