Effective Haskell

Deriving Instances

You’ve seen how type classes allow you to create abstractions in your programs. A problem with type classes that you might have already run into is that most of the time you’re doing a lot of work to create instances, but the code is uninteresting and often redundant. As an example, let’s imagine a simple record to hold some customer info:

data Customer = Customer
  { name  :: String
  , mail  :: String
  , email :: String
  }

For our application, we’d like to provide implementations for three common type classes, Show, which will allow us to convert a customer record to a string, Eq, which will let us test equality between two records, and Ord, which will allow us to sort our records. We can manually implement ...