Classes.hs

class A a where
    getB :: B b => a -> b

class B b where
    getA :: A a => b -> a

data X = X

instance A X where
    getB x = x

instance B X where
    getA x = x
    
{-    
Foo.hs:16:14:
    Could not deduce (b ~ X)
    from the context (B b)
      bound by the type signature for getB :: B b => X -> b
      at Foo.hs:16:5-14
      `b' is a rigid type variable bound by
          the type signature for getB :: B b => X -> b at Foo.hs:16:5
    In the expression: x
    In an equation for `getB': getB x = x
    In the instance declaration for `A X'

Foo.hs:19:14:
    Could not deduce (a ~ X)
    from the context (A a)
      bound by the type signature for getA :: A a => X -> a
      at Foo.hs:19:5-14
      `a' is a rigid type variable bound by
          the type signature for getA :: A a => X -> a at Foo.hs:19:5
    In the expression: x
    In an equation for `getA': getA x = x
    In the instance declaration for `B X'
-}