Re: To allow deriving poly-kinded Generic1 instances

Currently, GHC rejects the following code:

{-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE PolyKinds #-} import GHC.Generics

data HKD f = Foo (f Int) (f Double) | Bar (f Bool) deriving Generic1 The compilation error is

• Can't make a derived instance of ‘Generic1 HKD’: Constructor ‘Foo’ applies a type to an argument involving the last

Thanks for posting an example—that's very helpful to figure out what is going on. parameter

but the applied type is not of kind * -> *, and Constructor ‘Foo’ applies a type to an argument involving the last

parameter

but the applied type is not of kind * -> *, and Constructor ‘Bar’ applies a type to an argument involving the last

parameter

but the applied type is not of kind * -> * • In the data declaration for ‘HKD’

I see. That error message could be worded better, in my opinion. The issue really isn't so much about the kind of `f`. If you had written `data HKD (f :: Type -> Type) = Baz (Proxy f)`, for instance, I would expect it to work. The real issue is _where_ `f` appears in the data constructors. In `Bar`, for instance, you have:

Bar (f Bool)

`Generic1` is limited to data types where the last type parameter only appears as the last type argument in any field. This means that a field like `Proxy f` would be fine, as that would be represented as `Rec1 Proxy` in a `Rep1` instance. `f Bool`, on the other hand, is problematic. `GHC.Generics` doesn't have a representation type that simultaneously allows representing this field while also "focusing" on the last type parameter like `Rec1`, `Par1`, etc. would allow. The `Foo` constructor has similar issues. The error-reporting machinery for `DeriveGeneric` essentially just accumulates every issue it encounters and reports everything at once, which is why there is a duplicate error message involving `Foo`. Needless to say, this kind of error message could be improved.

Although it is possible to define a hand-rolled instance of Generic1

Really? I'm not sure how you would define a correct `Generic1` instance for `HKD` at all. What did you have in mind? Best, Ryan On Wed, Oct 13, 2021 at 8:33 AM Fumiaki Kinoshita wrote:

Currently, GHC rejects the following code:

{-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE PolyKinds #-} import GHC.Generics

data HKD f = Foo (f Int) (f Double) | Bar (f Bool) deriving Generic1

The compilation error is

• Can't make a derived instance of ‘Generic1 HKD’: Constructor ‘Foo’ applies a type to an argument involving the last parameter but the applied type is not of kind * -> *, and Constructor ‘Foo’ applies a type to an argument involving the last parameter but the applied type is not of kind * -> *, and Constructor ‘Bar’ applies a type to an argument involving the last parameter but the applied type is not of kind * -> * • In the data declaration for ‘HKD’ | 7 | deriving Generic1

Although it is possible to define a hand-rolled instance of Generic1, DeriveGeneric is still restricted to Type -> Type.

2021年10月13日(水) 21:17 Ryan Scott :

...

Hello,

I'm not quite sure I understand the issue you're hitting. Generic1 is poly-kinded, so I would expect it to be able to handle data types where the last type parameter has differing kinds. Can you post a complete example of the program you expect to typecheck, but doesn't?

Best,

Ryan _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs