Reverse data constructor

Question

How can I define a function that will accept my type and return its primitive "synonym"? For instance:

newtype MyInt = MakeInt Int

And I want a function:

unMyInt :: MakeInt -> Int

Answer 1

By figurative comparison of the constructor:

unMyInt (MakeInt i) = i

Answer 2

Another (and more convenient way sometimes) is to write syntax:

newtype myInt a = MyInt { unMyInt :: Int }

This automatically determines the function

unMyInt :: MyInt -> Int