unit impl where
    module A where
        data T = T
    module B(module A, module B) where
        import A
        instance Show T where
            show T = "T"
    module C(module B) where
        import B

unit sig where
    signature B where
        data T
        instance Show T
    module App where
        import B
        app :: T -> IO ()
        app t = print t

unit top where
    dependency sig[B=impl:C]