Question about Roles and the AMP work

Edward Kmett ekmett at
Mon May 12 15:28:56 UTC 2014

As a straw man, if we went to a typeclass-driven 'next arg is
representational' approach then we could possibly recover 'join' here.


class Representational f where
  rep :: Coercion f g -> Coercion a b -> Coercion (f a) (g b)

class Representational f => Phantom f where
  phantom :: Coercion f g -> Coercion (f a) (g b)

When we look at for the constraint head Coercible (f a) (g b) we'd then
check for Phantom f and if so look for Coercible f g, or fall back to
f, and look for Coercible f g and Coercible a b.

Then to derive GND for C m it'd need to check Representational m due to the
type of join needing m's next arg to be representational, find it and move

The set of these instances can be left open like any other typeclass and
users could supply their own for tricky conversions if need be.

If you just wrote the existing role annotations, but let them request
currently impossible roles you could use the desired role to infer the
constraints involved.


newtype StateT s m a = StateT { runStateT :: s -> m (a, s) }

should be able to derive something like

instance Representational (StateT s)
instance Representational m => Representational (StateT s m)
instance Phantom m => Phantom (StateT s m)

If we put nominal or representational on the m argument

type role StateT nominal nominal nominal

then we could just elide the instances for anything lower in the nominal ->
representational -> phantom lattice than the named role.

The positional argument scheme isn't perfect, as in the above it'd fail to
let you derive `instance Representational StateT` due to the s occurring
under m, an argument later in the argument list, but it does fix most of
the problems with Coercible and monad transfomers.

There might be some fancier form that can cover all of the use cases, but
I'm not seeing it off hand.


On Tue, May 13, 2014 at 12:59 AM, Simon Peyton Jones
<simonpj at>wrote:

> This looks a bit awkward to me.  Here's a small test case
> class C m where
>   ret :: a -> m a
>   bind :: m a -> (a -> m b) -> m b
>   join :: m (m a) -> m a
> newtype T m a = MkT (m a) deriving( C )
> This is accepted without the 'join' in the class, but rejected with it.
> So *all* uses of GND to do 'deriving( Monad )' will break when 'join' is
> added to Monad.
> And this is because we don't know that 'm' ranges only over things that
> have representational arguments; type variables always have nominal
> arguments.  The paper draws attention to this shortcoming, but now it's
> becoming important.
> I can't think of an obvious workaround either, except not putting 'join'
> in Monad.
> Simon
> | -----Original Message-----
> | From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Austin
> | Seipp
> | Sent: 12 May 2014 13:53
> | To: ghc-devs at
> | Subject: Question about Roles and the AMP work
> |
> | Hello all,
> |
> | While picking the AMP patch back up, it is almost ready to go, but I got
> | quite a puzzling error when attempting to build 'haskeline' with the AMP
> | changes. I don't seem to remember encountering this error before - but
> | maybe my mind is hazy.
> |
> | The error in question is this:
> |
> |
> |
> | The TL;DR is that the move of `join` into base has caused a problem
> | here. The error really speaks for itself: GND tries to derive an
> | instance of `join` for DumbTerm, which would have the type
> |
> |   join :: DumbTerm m (DumbTerm m a) -> DumbTerm m a
> |
> | and be equivalent to join for StateT. But this can't hold: DumbTerm
> | should unwrap directly to StateT (by newtype equality) and thus the
> | third parameter to StateT in this case is `DumbTerm m a` if you expand
> | the types - but because this argument is Nominal, it cannot unify with
> | 'PosixT m', which is the actual definition of DumbTerm in the first
> | place!
> |
> | With a little thought, this error actually seems reasonable. But it
> | puzzles me as to what we should do now. One thing we could do is just
> | write the instances manually, of course. But that's a bit annoying.
> |
> | Another alternative (and also unfortunate) change is to leave `join` out
> | of Monad, in which case the GND mechanism is oblivious to it and does
> | not need to worry about deriving these cases.
> |
> | I am not sure how many packages depend on using GND to derive monads
> | like StateT with Nominal arguments, but I imagine it is not totally
> | insignificant and will essentially hurt a lot of people who may use it.
> |
> | Richard, I'm particularly interested to hear what you think. This one
> | sort of snuck up!
> |
> | (NB: despite this, the AMP work is very close to landing in HEAD, at
> | which point we can begin cleaning stuff up faster.)
> |
> | --
> | Regards,
> |
> | Austin Seipp, Haskell Consultant
> | Well-Typed LLP,
> | _______________________________________________
> | ghc-devs mailing list
> | ghc-devs at
> |
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