Yes, thats good solution. Let me complicate things a bit. Lets define last data, D, in a different way:

data A a = A Int Int (B a) data B a = B String String (C a) data C a = C Int Int (D a)

data D a = D Int (F a)

where F is a type function

type family F t data Stage1 data Stage2 type instance F Stage1 = Int type instance F Stage2 = String

This time D is not a functor, because 'a' can accept only either Stage1 or Stage2. And what I need is to quickly define copy :: A Stage1 -> A Stage2 is it possible without writing boilerplate? 2012/10/2 Erik Hesselink :

You could add {-# LANGUAGE DeriveFunctor #-}, and then add 'deriving Functor' to all your data types (or you could of course manually define your functor instances). Then what you want is just 'fmap show'.

Erik

On Tue, Oct 2, 2012 at 9:55 AM, Sergey Mironov wrote:

...

Hi! I have a syntax tree defined like this:

...

data A a = A Int Int (B a)

data B a = B String String (C a)

data C a = C Int Int (D a)

and so on, all the data are parametrized with a type variable. This variable is actually used as a field in the very end of a hierarchy:

...

data D a = D Int a

Now I have to write a function which would copy (A Int) to (A String). Is it possible to do so using TH/syb without writing

copyA (A i1 i2 b) = A i1 i2 (copyB b) copyB = ... copyC = ... .. copyD (D i a) = D i (show a) ?

Could you provide me with a hint?

Thanks, Sergey

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