Re: More GND + role inference woes

Yes, I believe that's right. As far as I can figure out, these classes really *are* problematic, in that if we allowed GeneralizedNewtypeDeriving for them, there would be a way to subvert the type system. To make these derivable, we would need to be able to restrict various type parameters from taking on values that take a nominal argument. Without the ability to restrict the values in this way, there could be trouble. Richard On Dec 14, 2013, at 4:52 PM, Ben Gamari wrote:

Edward Kmett writes:

...

If this forced me to write those instances by hand, I could accept that as a tax for correctness. It means you can't GND any of the HasFoo dictionaries that lens builds, but meh.

Am I correct in assuming that Bind, R1, R2, R3, and R4 are the problematic instances in linear? With recent GHC I get the errors below.

Cheers,

- Ben

src/Linear/Affine.hs:112:34: Could not coerce from ‛f (f a)’ to ‛f (Point f a)’ because ‛f (f a)’ and ‛f (Point f a)’ are different types. arising from the coercion of the method ‛join’ from type ‛forall a. f (f a) -> f a’ to type ‛forall a. Point f (Point f a) -> Point f a’ Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (Bind (Point f))

src/Linear/Affine.hs:112:58: Could not coerce from ‛g (f x)’ to ‛g (Point f x)’ because ‛g (f x)’ and ‛g (Point f x)’ are different types. arising from the coercion of the method ‛core’ from type ‛forall a. ((forall (g :: * -> *) x. Functor g => (x -> g x) -> f x -> g (f x)) -> a) -> f a’ to type ‛forall a. ((forall (g :: * -> *) x. Functor g => (x -> g x) -> Point f x -> g (Point f x)) -> a) -> Point f a’ Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (Core (Point f))

src/Linear/Affine.hs:112:64: Could not coerce from ‛f1 (f a)’ to ‛f1 (Point f a)’ because ‛f1 (f a)’ and ‛f1 (Point f a)’ are different types. arising from the coercion of the method ‛_x’ from type ‛forall a (f :: * -> *). Functor f => (a -> f a) -> f a -> f (f a)’ to type ‛forall a (f :: * -> *). Functor f => (a -> f a) -> Point f a -> f (Point f a)’ Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (R1 (Point f))

src/Linear/Affine.hs:112:68: Could not coerce from ‛f1 (f a)’ to ‛f1 (Point f a)’ because ‛f1 (f a)’ and ‛f1 (Point f a)’ are different types. arising from the coercion of the method ‛_xy’ from type ‛forall a (f :: * -> *). Functor f => (V2 a -> f (V2 a)) -> f a -> f (f a)’ to type ‛forall a (f :: * -> *). Functor f => (V2 a -> f (V2 a)) -> Point f a -> f (Point f a)’ Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (R2 (Point f))

src/Linear/Affine.hs:112:68: Could not coerce from ‛f1 (f a)’ to ‛f1 (Point f a)’ because ‛f1 (f a)’ and ‛f1 (Point f a)’ are different types. arising from the coercion of the method ‛_y’ from type ‛forall a (f :: * -> *). Functor f => (a -> f a) -> f a -> f (f a)’ to type ‛forall a (f :: * -> *). Functor f => (a -> f a) -> Point f a -> f (Point f a)’ Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (R2 (Point f))

src/Linear/Affine.hs:112:72: Could not coerce from ‛f1 (f a)’ to ‛f1 (Point f a)’ because ‛f1 (f a)’ and ‛f1 (Point f a)’ are different types. arising from the coercion of the method ‛_xyz’ from type ‛forall a (f :: * -> *). Functor f => (V3 a -> f (V3 a)) -> f a -> f (f a)’ to type ‛forall a (f :: * -> *). Functor f => (V3 a -> f (V3 a)) -> Point f a -> f (Point f a)’ Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (R3 (Point f))

src/Linear/Affine.hs:112:72: Could not coerce from ‛f1 (f a)’ to ‛f1 (Point f a)’ because ‛f1 (f a)’ and ‛f1 (Point f a)’ are different types. arising from the coercion of the method ‛_z’ from type ‛forall a (f :: * -> *). Functor f => (a -> f a) -> f a -> f (f a)’ to type ‛forall a (f :: * -> *). Functor f => (a -> f a) -> Point f a -> f (Point f a)’ Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (R3 (Point f))

src/Linear/Affine.hs:112:76: Could not coerce from ‛f1 (f a)’ to ‛f1 (Point f a)’ because ‛f1 (f a)’ and ‛f1 (Point f a)’ are different types. arising from the coercion of the method ‛_xyzw’ from type ‛forall a (f :: * -> *). Functor f => (V4 a -> f (V4 a)) -> f a -> f (f a)’ to type ‛forall a (f :: * -> *). Functor f => (V4 a -> f (V4 a)) -> Point f a -> f (Point f a)’ Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (R4 (Point f))

src/Linear/Affine.hs:112:76: Could not coerce from ‛f1 (f a)’ to ‛f1 (Point f a)’ because ‛f1 (f a)’ and ‛f1 (Point f a)’ are different types. arising from the coercion of the method ‛_w’ from type ‛forall a (f :: * -> *). Functor f => (a -> f a) -> f a -> f (f a)’ to type ‛forall a (f :: * -> *). Functor f => (a -> f a) -> Point f a -> f (Point f a)’ Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (R4 (Point f))