omit-generics

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omit-generics =omit-generics= aims to make deriving ~Eq~ and ~Ord~ instances easier by giving the programmer control over what fields to ignore.

This is best explained by an example: #+BEGIN_SRC haskell :multi-line t {-# LANGUAGE DataKinds #-} {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE DerivingStrategies #-} {-# LANGUAGE DerivingVia #-}

import GHC.Generics import GHC.Generics.Omit

data Person = Person { name :: String, age :: Int, metadata :: [String] } deriving stock Generic deriving Eq via (Omit '["age", "metadata"] Person) #+END_SRC

Now, when we compare ~Person~ for equality, the ~age~ and ~metadata~ fields are ignored!

#+BEGIN_SRC haskell Person "Steve" 43 ["loves cats"] == Person "Steve" 1 ["loves dogs", "is a baby"]

True #+END_SRC