{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}

module Foo1 where

import Data.Foldable

class Foo t where
  type FooPType t :: *              -- Base type.
  type FooFType t :: * -> *         -- Container type.
  defPs :: FooFType t (FooPType t)  -- Initialized container.

-- An attempt at a universal testing function valid for all types, t,
-- of class Foo for which `FooFType t` is a foldable functor.
tst :: forall t.
  ( Foo t
  , Functor  (FooFType t)
  , Foldable (FooFType t)
  ) => FooPType t
tst = (head . toList) defPs

Foo1.hs:30:23: error:
    • Couldn't match type ‘FooPType t0’ with ‘FooPType t’
      Expected type: FooFType t0 (FooPType t)
        Actual type: FooFType t0 (FooPType t0)
      NB: ‘FooPType’ is a type function, and may not be injective
      The type variable ‘t0’ is ambiguous
    • In the first argument of ‘head . toList’, namely ‘defPs’
      In the expression: (head . toList) defPs
      In an equation for ‘tst’: tst = (head . toList) defPs
    • Relevant bindings include
        tst :: FooPType t (bound at Foo1.hs:30:1)