“Pattern matching” of algebraic type data constructors

Question

Let\'s consider a data type with many constructors:

data T = Alpha Int | Beta Int | Gamma Int Int | Delta Int

I want to write a function to che

Answer 1

You'll need to use a generics library like Scrap Your Boilerplate or uniplate to do this in general.

If you don't want to be so heavy-handed, you can use Dave Hinton's solution, together with the empty record shortcut:

...
where f (Alpha {}) = 0
      f (Beta {}) = 1
      f (Gamma {}) = 2

So you don't have to know how many args each constructor has. But it obviously still leaves something to be desired.