Param traversal fails for nested lists (also nested Maybes etc)

[czechow-shadow]

I just stumbled upon what looks like inconsistent behavior of auto derived traversal and generic-lens while employing nested lists (deep traversal?). Minimal example below (tested with generic-lens 1.1.0.0 and ghc 8.2.2):

{-# LANGUAGE DataKinds         #-}
{-# LANGUAGE DeriveFoldable    #-}
{-# LANGUAGE DeriveFunctor     #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE DeriveTraversable #-}
{-# LANGUAGE TypeApplications  #-}

module Lib where

import GHC.Generics

import Data.Generics.Product
import Control.Lens (traverseOf)

data MT a = MT [[a]] deriving (Show, Generic, Functor, Foldable, Traversable)

mt :: MT Int
mt = MT [[1]]

go :: IO ()
go = do
  correct <- traverse (\x -> pure $ succ x) mt
  wrong <- traverseOf (param @0) (\x -> pure $ succ x) mt

  putStrLn $ "Traversal results differ: " ++ show correct ++ " vs " ++ show wrong

We expect the results to be MT [[2]] in both cases, but generic-lens outputs MT [[1]].

[balajirrao]

I've seen a version of this. In this case it fails to compile with generic-lens 1.1.0.0 on ghc 8.6.3

{-# LANGUAGE DataKinds         #-}
{-# LANGUAGE DeriveFoldable    #-}
{-# LANGUAGE DeriveFunctor     #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE DeriveTraversable #-}
{-# LANGUAGE TypeApplications  #-}

module GenericLensMinimalExample where

import GHC.Generics (Generic)
import Data.Generics.Product (param, Rec(..))
import Control.Lens (over)

data Foo a = Foo a deriving (Eq, Show, Generic)
data Bar b = Bar b deriving (Eq, Show, Generic)
data FooBar c = FooBar (Foo (Bar c)) deriving (Eq, Show, Generic)

foo :: FooBar Int -> FooBar String
foo = over (param @0) show

GenericLensMinimalExample.hs:19:13: error:
    • Couldn't match type ‘Int’ with ‘[Char]’
        arising from a functional dependency between:
          constraint ‘Data.Generics.Product.Param.GHasParamRec
                        'Nothing (Foo (Bar Int)) (Foo (Bar [Char])) Int [Char]’
            arising from a use of ‘param’
          instance ‘Data.Generics.Product.Param.GHasParamRec
                      'Nothing a a c d’
            at <no location info>
    • In the first argument of ‘over’, namely ‘(param @0)’
      In the expression: over (param @0) show
      In an equation for ‘foo’: foo = over (param @0) show