[Haskell-cafe] Strange kind error when using Lens.traverseOf

[Henning Thielemann]

I have this simplified module:

module Ganeti.LensPlain where

import Control.Lens (LensLike, traverseOf)
import Data.Functor.Compose (Compose(..))

traverseOf2 ::
    LensLike (Compose f g) s t a b -> (a -> f (g b)) -> s -> f (g t)
traverseOf2 k f = getCompose . traverseOf k (Compose . f)


With ghc-9.2 and later I get this kind error:

src/Ganeti/LensPlain.hs:8:32: error: [GHC-25897]
     • Couldn't match kind ‘k2’ with ‘*’
       When matching types
         Compose f g0 :: * -> *
         Compose f g :: k2 -> *
       Expected: s -> Compose f g t
         Actual: s -> Compose f g0 t0
       ‘k2’ is a rigid type variable bound by
         the type signature for:
           traverseOf2 :: forall {k1} {k2} (f :: k1 -> *) (g :: k2 -> k1) s
                                 (t :: k2) a (b :: k2).
                          LensLike (Compose f g) s t a b -> (a -> f (g b)) 
-> s -> f (g t)
         at src/Ganeti/LensPlain.hs:7:1-79
     • In the second argument of ‘(.)’, namely
         ‘traverseOf k (Compose . f)’
       In the expression: getCompose . traverseOf k (Compose . f)
       In an equation for ‘traverseOf2’:
           traverseOf2 k f = getCompose . traverseOf k (Compose . f)
     • Relevant bindings include
         f :: a -> f (g b) (bound at src/Ganeti/LensPlain.hs:8:15)
         k :: LensLike (Compose f g) s t a b
           (bound at src/Ganeti/LensPlain.hs:8:13)
         traverseOf2 :: LensLike (Compose f g) s t a b
                        -> (a -> f (g b)) -> s -> f (g t)
           (bound at src/Ganeti/LensPlain.hs:8:1)
   |
8 | traverseOf2 k f = getCompose . traverseOf k (Compose . f)
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^



GHC-9.0 accepts the code. Also GHC-9.2 accepts it when the module is 
compiled as part of a larger Cabal project!

I thought that (.) would cause the problem, but manually inlining it does 
not help.


I compiled with:

$ ghc-9.6.6 -package lens -c src/Ganeti/LensPlain.hs