A protocol to define how elements of a type can be compared.

The simplest way to define it for a custom struct is to use @derive based on semantical comparison of fields, in the given order:

defmodule MyStruct do
  @derive {Cmp.Comparable, using: [:date, :id]}

  defstruct [:id, :date]
end

Cmp.sort([
  %MyStruct{date: ~D[2020-03-02], id: 100},
  %MyStruct{date: ~D[2020-03-02], id: 101},
  %MyStruct{date: ~D[2019-06-06], id: 300}
])
[
  %MyStruct{date: ~D[2019-06-06], id: 300},
  %MyStruct{date: ~D[2020-03-02], id: 100},
  %MyStruct{date: ~D[2020-03-02], id: 101}
]

For existing structs that are already implementing a compare/2 function returning :eq | :lt | :gt, the protocol can simply be done by passing using: :compare.

require Protocol
Protocol.derive(Cmp.Comparable, MyStruct, using: :compare)

This is already done for the following modules:

• Date
• Time
• DateTime
• NaiveDateTime
• Version
• Decimal (if available)