Code generated for non-regular recursive types does not typecheck

[frerich]

Using catamorphism 0.5.0.1, the following program fails to compile:

{-# LANGUAGE TemplateHaskell #-}
import Data.Morphism.Cata

data List a = Empty | Cons a (List (List a))

$(makeCata defaultOptions ''List)

main = return ()

The makeCata invocation generates code yielding the error message

treecata.hs:6:3:
    Couldn't match type ‘a1’ with ‘List a1’
      ‘a1’ is a rigid type variable bound by
           the type signature for list :: a -> (a1 -> a -> a) -> List a1 -> a
           at treecata.hs:6:3
    Expected type: List a1
      Actual type: List (List a1)
    Relevant bindings include
      a_a2p8 :: List (List a1) (bound at treecata.hs:6:3)
      a_a2p7 :: a1 (bound at treecata.hs:6:3)
      x_a2p6 :: List a1 (bound at treecata.hs:6:3)
      c_a2p5 :: a1 -> a -> a (bound at treecata.hs:6:3)
      list :: a -> (a1 -> a -> a) -> List a1 -> a
        (bound at treecata.hs:6:3)
    In the third argument of ‘list’, namely ‘a_a2p8’
    In the second argument of ‘c_a2p5’, namely
      ‘list c_a2p4 c_a2p5 a_a2p8’

[frerich]

The answers to my question on StackOverflow suggest that this is actually quite tricky.