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I have problems deriving Show and Eq instances for a data class when following the instructions here: https://github.com/fumieval/barbies-th?tab=readme-ov-file#deriving-pass-through

I added these two test cases to the test suite of barbies-th and they also fail to come up with the according instances.

test_show :: String
test_show = show test_con

test_eql :: Bool
test_eql = test_con == test_con

Do I have a problem understanding how to derive the instances with barbies-th or is the functionality broken at the moment?

Currently, if I want to use Barbies-TH to create a HKD that has per-field Semigroup and Monoid instances, I can do it like so:

declareBareB [d|
  data CPUOut = CPUOut
      { progAddr :: PC
      , memAddr :: Ptr
      , memWrite :: Maybe Cell
      , output :: Maybe Cell
      , inputNeeded :: Bool
      }|]
deriving via Barbie (CPUOut Covered) f instance (forall a. Semigroup (f a)) => Semigroup (CPUOut Covered f)
deriving via Barbie (CPUOut Covered) f instance (forall a. Monoid (f a)) => Monoid (CPUOut Covered f)

Would it be possible to automate this / hide the deriving via magic, and allow the user to instead write

declareBareB [d|
  data CPUOut = CPUOut
      { progAddr :: PC
      , memAddr :: Ptr
      , memWrite :: Maybe Cell
      , output :: Maybe Cell
      , inputNeeded :: Bool
      }
      deriving (Semigroup, Monoid)|]

which would generate the same code as above?

barbies-th works fine with DuplicateRecordFields enabled (thanks to the name unmangling patches); but it can't handle record fields that are actually duplicated. Possibly this is an accepted limitation, but it would be nice to document it I think. The error message is not very friendly:

... Assembler messages: ... Error: symbol `testlibzm0zi1zi0zi0zminplace_Lib_qux_info' is already defined ... testlibzm0zi1zi0zi0zminplace_Lib_qux_info

@Elvecent reported this as well, at the bottom of the thread in #3.

GHC 8.10.7
barbies == 2.0.3.1
barbies-th == 0.1.10

Example:

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DuplicateRecordFields #-}

module Lib where

import Barbies.TH (declareBareB)

declareBareB
  [d|
    data Foo = Foo
      { qux :: Integer
      }
    |]

declareBareB
  [d|
    data Bar = Bar
      { qux :: Integer
      }
    |]

cabal-version:   3.4
name:            testlib
version:         0.1.0.0
build-type:      Simple

common warnings
    ghc-options: -Wall

library
    import:           warnings
    exposed-modules:  Lib
    build-depends:
        base ^>=4.14.3.0,
        barbies == 2.0.3.1,
        barbies-th == 0.1.10
    hs-source-dirs:   src
    default-language: Haskell2010

peter@gtower:~/2022october8_reproduce_barbies_error$ cabal build
Build profile: -w ghc-8.10.7 -O1
In order, the following will be built (use -v for more details):
 - testlib-0.1.0.0 (lib) (first run)
Preprocessing library for testlib-0.1.0.0..
Building library for testlib-0.1.0.0..
[1 of 1] Compiling Lib              ( src/Lib.hs, /home/peter/2022october8_reproduce_barbies_error/dist-newstyle/build/x86_64-linux/ghc-8.10.7/testlib-0.1.0.0/build/Lib.o, /home/peter/2022october8_reproduce_barbies_error/dist-newstyle/build/x86_64-linux/ghc-8.10.7/testlib-0.1.0.0/build/Lib.dyn_o )
/tmp/ghc80775_0/ghc_2.s: Assembler messages:

/tmp/ghc80775_0/ghc_2.s:1298:0: error:
     Error: symbol `testlibzm0zi1zi0zi0zminplace_Lib_qux_info' is already defined
     |
1298 | testlibzm0zi1zi0zi0zminplace_Lib_qux_info:
     | ^

/tmp/ghc80775_0/ghc_2.s:1329:0: error:
     Error: symbol `testlibzm0zi1zi0zi0zminplace_Lib_qux_closure' is already defined
     |
1329 | testlibzm0zi1zi0zi0zminplace_Lib_qux_closure:
     | ^
`gcc' failed in phase `Assembler'. (Exit code: 1)

I tried to minimize the testcase further by expanding the splices and minimizing the generated code, but I was unable to achieve further minimization. I expanded one splice using the code-action offered by HLS. I found that the generated code is referencing a symbol called (Unit :: (Type -> Constraint) -> Type -> Constraint). I stubbed out such a type like this:

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE UndecidableSuperClasses #-}
import Data.Kind (Constraint, Type)
class c t => Unit (c :: Type -> Constraint) (t :: Type) where
  {- empty body -}

When I added that bit, the generated code started compiling. That is, without the "symbol already defined" error.

Thanks for the library, seems cool.

It would be nice to be able to do something like this:

declareBareB [d|
  data Foo = Foo { foo :: Int }
  data Bar = Bar { bar :: Foo }
  |]

And have both Foo and Bar become barbies, with Foo inheriting the clothing style form Bar

Example source illustrating the problem:

{-# LANGUAGE TemplateHaskell, FlexibleInstances, DeriveGeneric #-}
{-# LANGUAGE DuplicateRecordFields #-}
import Barbies
import Data.Barbie.TH

declareBareB [d|
  data R = MkR
      { foo :: Int
      } |]

This fails with:

src/Foo.hs:6:1: error:
    Illegal variable name: ‘$sel:foo:MkR’
    When splicing a TH declaration:
      data R_0 sw_1 h_2
    = MkR_3 {$sel:foo:MkR_4 :: (Barbies.Internal.Wear.Wear sw_1
                                                           h_2
                                                           GHC.Types.Int)}
    deriving GHC.Generics.Generic
  |
6 | declareBareB [d|
  | ^^^^^^^^^^^^^^^^...