12 Mar 2022

haskell deriving best practice

1. Haskell Typeclass Instances

Like it or hate it, you need to define a lot of typeclass instances when you define haskell types. The easy way to mitigate this annoying problem is to derive instances for the defined types. Following is some best practice for deriving instances.

1.1. What to derive

The first question is what instances to derive for the defined type. Following is a simple guileline:

For any types, the Show, Typeable, Generic instance should be derived
For sum types, the Ord, Eq should be derived
For Read instance, it depends

1.2. Deriving stratege

Next question is how to derive those instances for the defined types. Following summarize the guideline:

Use stock stratege to derive some standard type classes, like Show, Ord, Eq, Typeable and Generic
Use newtype stratege to derive instances for newtype definition