Yeah, this is definitely my favorite new library. The primary gain from it for me is making things DRYer. Where before I would have map structures repeated in multiple function clauses, e.g.
def handle_cmd(%{"dest_ib" => dest_ib,
"context_ib_gib" => context_ib_gib,
"src_ib_gib" => src_ib_gib} = data, ..) when guard1 do
def handle_cmd(%{"dest_ib" => dest_ib,
"context_ib_gib" => context_ib_gib,
"src_ib_gib" => src_ib_gib} = data, ..) when guard2 do
which has redundancy in the "var_name" => var_name, as well in the function clause level. And dest_ib and src_ib_gib are used across multiple commands in many places. I’m now able to create a single file for the reused patterns:
defmodule WebGib.Patterns do
@moduledoc """
Reusable patterns using expat
"""
import Expat
defpat dest_ib_ %{"dest_ib" => dest_ib}
defpat src_ib_gib_ %{"src_ib_gib" => src_ib_gib}
defpat context_ib_gib_ %{"context_ib_gib" => context_ib_gib}
# ..
end
(NB: I am tacitly going with a _ suffix to indicate a pattern vs the var name, but I’m not sure what other non-word characters are legal in elixir. I would rather prefix the pattern with a single character and would love any suggestions.)
And then I compose them above the function and consume them:
defpat fork_data_(
dest_ib_() =
context_ib_gib_() =
src_ib_gib_()
)
def handle_cmd(fork_data_(...) = data, ..) when guard1 do
def handle_cmd(fork_data_(...) = data, ..) when guard1 do
EDIT: The ... inside the pattern is literal syntax, which helps enormously with DRY. The other .. just means other args.
This is ridiculously more DRY and readable. Definitely a powerful lib you made here! 
Thank you! 
