Error found:
in module FoldableInstance9
at tests/purs/failing/FoldableInstance9.purs:53:1 - 53:38 (line 53, column 1 - line 53, column 38)

  One or more type variables are in positions that prevent Foldable from being derived.
  To derive this class, make sure that these variables are only used as the final arguments to type constructors, and that those type constructors themselves have instances of Foldable.

  tests/purs/failing/FoldableInstance9.purs:
     15 data Test f g h a
     16   = Test1 (f a a a) (f Int a a) (f a a Int) (f Int a Int) (f Int Int Int)
     17   | Test2 { all :: f a a a
     18           , rights :: f Int a a
     19           , lefts :: f a a Int
     20           , middle :: f Int a Int
     21           , none :: f Int Int Int
     22           }
     23   | Test3 (g
     24             { all :: f a a a
     25             , rights :: f Int a a
     26             , lefts :: f a a Int
     27             , middle :: f Int a Int
     28             , none :: f Int Int Int
    ...
     32             { nested1 ::
     33                 { all :: f a a a
     34                 , rights :: f Int a a
     35                 , lefts :: f a a Int
     36                 , middle :: f Int a Int
     37                 , none :: f Int Int Int
    ...
     40                 g
     41                   { all :: f a a a
     42                   , rights :: f Int a a
     43                   , lefts :: f a a Int
     44                   , middle :: f Int a Int
     45                   , none :: f Int Int Int
     46                   }
     47                   a
     48             }
     49             a)
     50   | Test5 (Rec f a)
     51   | Test6 (g (Rec f a) a)
     52   | Test7 (h { nested1 :: Rec f a, nested2 :: g (Rec f a) a } a)
     53 derive instance Foldable (Test f g h)


See https://github.com/purescript/documentation/blob/master/errors/CannotDeriveInvalidConstructorArg.md for more information,
or to contribute content related to this error.