Fails to build on GHC 8.2

[RyanGlScott]

When doing some field testing today, I noticed that swagger2 fails to build with GHC 8.2:

$ cabal build
Building swagger2-2.1.3...
Preprocessing library swagger2-2.1.3...
[ 9 of 14] Compiling Data.Swagger.Internal.Schema ( src/Data/Swagger/Internal/Schema.hs, dist/build/Data/Swagger/Internal/Schema.o )

src/Data/Swagger/Internal/Schema.hs:638:25: error:
    • Overlapping instances for GSumToSchema (C1 c U1)
        arising from a use of ‘gdeclareNamedSumSchema’
      Matching instances:
        two instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the expression: gdeclareNamedSumSchema
      In an equation for ‘gdeclareNamedSchema’:
          gdeclareNamedSchema = gdeclareNamedSumSchema
      In the instance declaration for ‘GToSchema (C1 c U1)’
    |
638 |   gdeclareNamedSchema = gdeclareNamedSumSchema
    |                         ^^^^^^^^^^^^^^^^^^^^^^

src/Data/Swagger/Internal/Schema.hs:763:84: error:
    • Couldn't match kind ‘k’ with ‘*’
      ‘k’ is a rigid type variable bound by
        the instance declaration
        at src/Data/Swagger/Internal/Schema.hs:762:10-48
      When matching the kind of ‘proxy’
      Expected type: proxy (C1 c U1)
        Actual type: proxy (C1 c U1)
    • In the third argument of ‘gsumConToSchemaWith’, namely ‘proxy’
      In the second argument of ‘(.)’, namely
        ‘gsumConToSchemaWith (Inline nullarySchema) opts proxy’
      In the expression:
        pure . gsumConToSchemaWith (Inline nullarySchema) opts proxy
    • Relevant bindings include
        proxy :: proxy (C1 c U1)
          (bound at src/Data/Swagger/Internal/Schema.hs:763:21)
        gsumToSchema :: SchemaOptions
                        -> proxy (C1 c U1)
                        -> Schema
                        -> WriterT AllNullary (Declare (Definitions Schema)) Schema
          (bound at src/Data/Swagger/Internal/Schema.hs:763:3)
    |
763 |   gsumToSchema opts proxy = pure . gsumConToSchemaWith (Inline nullarySchema) opts proxy
    |                                                                                    ^^^^^

The issue is that starting in GHC 8.2, the datatypes in GHC.Generics became poly-kinded. There are two ways one could fix this problem:

1. Use an explicit kind signature for GSumToSchema:

   diff --git a/src/Data/Swagger/Internal/Schema.hs b/src/Data/Swagger/Internal/Schema.hs
   index ae6a7b2..1f6b3dd 100644
   --- a/src/Data/Swagger/Internal/Schema.hs
   +++ b/src/Data/Swagger/Internal/Schema.hs
   @@ -727,7 +727,7 @@ gdeclareNamedSumSchema opts proxy s
    
    type AllNullary = All
    
   -class GSumToSchema f where
   +class GSumToSchema (f :: * -> *) where
      gsumToSchema :: SchemaOptions -> proxy f -> Schema -> WriterT AllNullary (Declare (Defi
    
    instance (GSumToSchema f, GSumToSchema g) => GSumToSchema (f :+: g) where

   This would be identical to inferred kind signature prior to GHC 8.2.

2. Generalize the kind signature of GToSchema:

   diff --git a/src/Data/Swagger/Internal/Schema.hs b/src/Data/Swagger/Internal/Schema.hs
   index ae6a7b2..d5965d5 100644
   --- a/src/Data/Swagger/Internal/Schema.hs
   +++ b/src/Data/Swagger/Internal/Schema.hs
   @@ -404,7 +404,7 @@ sketchStrictSchema = go . toJSON
          where
            names = HashMap.keys o
    
   -class GToSchema (f :: * -> *) where
   +class GToSchema (f :: k -> *) where
      gdeclareNamedSchema :: SchemaOptions -> proxy f -> Schema -> Declare (Definitions Schem
    
    instance OVERLAPPABLE_ ToSchema a => ToSchema [a] where

Which option sounds better to you?

[phadej]

I assume both variants works with GHC>=7.8? I have no opinion on this, I assume generics functors are always instantiated as * -> * when used in deriving (Generic)?

[RyanGlScott]

I assume both variants works with GHC>=7.8?

Yes, both options are backwards compatible back to GHC 7.8.

I have no opinion on this, I assume generics functors are always instantiated as * -> * when used in deriving (Generic)?

Yes. It's only Generic1 that is allowed to take advantage of things of kind k -> *.