deriving.hs

-- this works
λ: data Foo t a = Foo (t a)
λ: :set -XStandaloneDeriving
λ: deriving instance (Show (t a), Show a) => Show (Foo t a)
λ: Foo ["test"]
Foo ["test"]

-- but this doesn't
λ: data Bar a = Bar a deriving Show; data Foo t a = Foo (t (Bar a))
λ: deriving instance (Show (t a), Show a) => Show (Foo t a)

<interactive>:15:1: error:
    • Could not deduce (Show (t (Bar a)))
        arising from a use of ‘showsPrec’
      from the context: (Show (t a), Show a)
        bound by the instance declaration at <interactive>:15:1-56
    • In the second argument of ‘(.)’, namely ‘(showsPrec 11 b1)’
      In the second argument of ‘showParen’, namely
        ‘((.) (showString "Foo ") (showsPrec 11 b1))’
      In the expression:
        showParen (a >= 11) ((.) (showString "Foo ") (showsPrec 11 b1))
      When typechecking the code for ‘showsPrec’
        in a derived instance for ‘Show (Foo t a)’:
        To see the code I am typechecking, use -ddump-deriv