How is Eq typeclass implemented for user defined types?

Question

For some user defined type such as the below how does the implementation of the Eq typeclass work? Its simple to write an implementation for things like Int or Float. But how is the catchall for all user types done since it would need to do things like pattern match against every possible value constructor? I'm not aware of any syntax to do this.

data Person = Person { firstName :: String
                     , lastName :: String
                     , age :: Int
                     } deriving (Eq)

Answer 1

It pattern matches against every possible value constructor, just like you said! For example, if you put your code in a file and run ghc with -ddump-deriv, here's what you get:

==================== Derived instances ====================
Derived instances:
  instance GHC.Classes.Eq Main.Person where
    GHC.Classes.==
      (Main.Person a1_alh a2_ali a3_alj)
      (Main.Person b1_alk b2_all b3_alm)
      = ((((a1_alh GHC.Classes.== b1_alk))
          GHC.Classes.&& ((a2_ali GHC.Classes.== b2_all)))
         GHC.Classes.&& ((a3_alj GHC.Classes.== b3_alm)))
    GHC.Classes./= a_aln b_alo
      = GHC.Classes.not ((GHC.Classes.==) a_aln b_alo)