Cmp

Semantic comparison and sorting for Elixir.

Why Cmp?

The built-in comparison operators as well as functions like Enum.sort/2 or Enum.max/1 are based on Erlang's term ordering and suffer two issues, which require attention and might lead to unexpected behaviors or bugs:

1. Structural comparisons

Built-ins use structural comparison over semantic comparison:

iex> ~D[2020-03-02] > ~D[2019-06-06]
false

iex> Enum.sort([~D[2019-01-01], ~D[2020-03-02], ~D[2019-06-06]])
[~D[2019-01-01], ~D[2020-03-02], ~D[2019-06-06]]

Semantic comparison is available but not straightforward:

iex> Date.compare(~D[2019-01-01], ~D[2020-03-02])
:lt

iex> Enum.sort([~D[2019-01-01], ~D[2020-03-02], ~D[2019-06-06]], Date)
[~D[2019-01-01], ~D[2019-06-06], ~D[2020-03-02]]

Cmp does the right thing out of the box:

iex> Cmp.gt?(~D[2020-03-02], ~D[2019-06-06])
true

iex> Cmp.sort([~D[2019-01-01], ~D[2020-03-02], ~D[2019-06-06]])
[~D[2019-01-01], ~D[2019-06-06], ~D[2020-03-02]]

2. Weakly typed

Built-in comparators accept any set of operands:

iex> 2 < "1"
true

iex> 0 < true
true

iex> false < nil
true

Cmp will only compare compatible elements or raise a Cmp.TypeError:

iex> Cmp.lte?(1, 1.0)
true

iex> Cmp.lte?(2, "1")
** (Cmp.TypeError) Failed to compare incompatible types - left: 2, right: "1"

Installation

Cmp can be installed by adding cmp to your list of dependencies in mix.exs:

def deps do
  [
    {:cmp, "~> 0.1.3"}
  ]
end

The documentation can be found at https://hexdocs.pm/cmp.

Design goals

• Fast and well-optimized - the overhead should be quite small over built-in equivalents. See the benchmarks/ folder for more details.
• No need to require macros, plain functions
• Easily extensible through the Cmp.Comparable protocol
• Robust and well-tested (both unit and property-based)

Supporting comparisons between non-homogeneous types such as mixed Decimal and built-in numbers for instance is a non-goal. This limitation is a necessary trade-off in order to ensure the points above. Use the Decimal library directly if you need this.

Copyright and License

Cmp is licensed under the MIT License.