It is assumed that instances of GHC.Generic class for data types should be generated automatically by GHC via deriving mechanism, but it is not working for row-types and anonymous records with implicit parameters. So, I am trying to define Generic instance manually and started the experiment with replicating Generic instance for a regular data type using GHCi to get generic representation, as preparation step, but even here I face unexplainable error from the type checker.

> import GHC.Generics
> data Point = Point { x :: Int, y :: Int } deriving (Show, Eq, Generic)
> from (Point 1 2)
M1 {unM1 = M1 {unM1 = M1 {unM1 = K1 {unK1 = 1}} :*: M1 {unM1 = K1 {unK1 = 2}}}}
> :i Rep Point
type instance Rep Point
  = D1
      (MetaData
         "Point"
         "RowTypeDemo.ImplicitParamsAsAlternativeToRowType"
         "row-type-demo-0.0.1-inplace"
         False)
      (C1
         (MetaCons "Point" PrefixI True)
         (S1
            (MetaSel
               (Just "x") NoSourceUnpackedness NoSourceStrictness DecidedLazy)
            (Rec0 Int)
          :*: S1
                (MetaSel
                   (Just "y") NoSourceUnpackedness NoSourceStrictness DecidedLazy)
                (Rec0 Int)))

Generic instance for Point based on GHCi output above:

data Point = Point { x :: Int, y :: Int } deriving (Show, Eq)
instance Generic Point where
  type Rep Point =
    D1
      (MetaData
         "Point"
         "RowTypeDemo.ImplicitParamsAsAlternativeToRowType"
         "row-type-demo-0.0.1-inplace"
         False)
      (C1
         (MetaCons "Point" PrefixI True)
         (S1
            (MetaSel
               (Just "x") NoSourceUnpackedness NoSourceStrictness DecidedLazy)
            (Rec0 Int)
          :*: S1
                (MetaSel
                   (Just "y") NoSourceUnpackedness NoSourceStrictness DecidedLazy)
                (Rec0 Int)))
  from :: Point -> Rep Point x
  from r = M1 {unM1 = L1 (M1 {unM1 = M1 {unM1 = K1 {unK1 = x r}} :*: M1 {unM1 = K1 {unK1 = y r}}})}

Type checker fails with:

• Couldn't match type: M1
                         i0 c0 (M1 i1 c1 (K1 i2 Int) :*: M1 i3 c2 (K1 i4 Int))
                       :+: g0
                 with: M1
                         C
                         (MetaCons "Point" PrefixI True)
                         (S1
                            (MetaSel
                               (Just "x") NoSourceUnpackedness NoSourceStrictness DecidedLazy)
                            (Rec0 Int)
                          :*: S1
                                (MetaSel
                                   (Just "y")
                                   NoSourceUnpackedness
                                   NoSourceStrictness
                                   DecidedLazy)
                                (Rec0 Int))
  Expected: Rep Point x
    Actual: M1
              D
              (MetaData
                 "Point"
                 "RowTypeDemo.ImplicitParamsAsAlternativeToRowType"
                 "row-type-demo-0.0.1-inplace"
                 False)
              (M1 i0 c0 (M1 i1 c1 (K1 i2 Int) :*: M1 i3 c2 (K1 i4 Int)) :+: g0)
              x
• In the expression:
    M1
      {unM1 = L1
                (M1
                   {unM1 = M1 {unM1 = K1 {unK1 = x r}}
                             :*: M1 {unM1 = K1 {unK1 = y r}}})}
  In an equation for ‘from’:
      from r
        = M1
            {unM1 = L1
                      (M1
                         {unM1 = M1 {unM1 = K1 {unK1 = x r}}
                                   :*: M1 {unM1 = K1 {unK1 = y r}}})}
  In the instance declaration for ‘Generic Point’    | 87 |   from r = M1 {unM1 = L1 (M1 {unM1 = M1 {unM1 = K1 {unK1 = x r}} :*: M1 {unM1

= K1 {unK1 = y r}}})} | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I don’t see where the problem is, because type family application Rep Point x should be equal to its value.

GHC version is 9.8.1

List of enabled extensions:

  DataKinds
  LambdaCase
  ImplicitParams
  OverloadedLabels
  OverloadedStrings
  TupleSections
  TypeFamilies
  UnicodeSyntax
  GADTs
  PolyKinds
  RankNTypes
  TypeOperators
  TypeApplications