[Haskell-cafe] arr f

Dear Haskellers, since this is my first post, "Hello all" I have a problem with a function taken from class Arrow. arr :: (b -> c) -> a b c To build my own arrow, i need to distinguish between different kinds of b or c. For instance, if b has the form (d,e), i want to construct something different as when having form (d,e,f). Could I get a hint how to express my needs, please! Many thanks in advance Sven Biedermann