@hackage database-id-class0.1.0.1

database-id-class

This package defines a (parametric) data type Id a used to refer to the primary key of values of type a in a database.

newtype Id a = Id { unId :: IdData a }

Here, IdData is a type family associated with the HasId class:

class HasId a where
  type IdData a :: *
  type IdData a = Int64

It defaults to Int64 because that tends to be the type you want for tables with an autoincrement primary key.