Expat - composable, reusable pattern matching

This is a really nice project. Since you are exploring things on the area, here are some challenges/questions you can consider:

1. Is it possible to extend expat to be a generalization of records? defpat contains an exact subset of the records functionality, which is the pattern matching one. It is also easy to see how defpat could be used for updates, all you need to do is to match on the value and then build another new value of exactly the same shape with the same variables in place except the ones you are replacing, etc.

2. Have you considered using another operator instead of = for mixing patterns? I believe your mixing of patterns is actually an intersection. If you assume that %{"id" => id} will match all maps with the "id" field, including %{"id" => "foo", "name" => "baz"}, and the pattern %{"name" => name} all maps with a "name" field, including the map mentioned above, when you specify id() = name() you are actually saying it should have BOTH patterns, id() AND name(). If you think of patterns as sets representing the structures they can match on, it is an intersection. Another reason to choose another operator is that the precedence will work in a way it won’t require parentheses, for example: defpat subject id() &&& name(). Here is a list of operators.

There is one feature we could add to Elixir that would allow the library to become more powerful which is to allow guards inside patterns:

def is_foo_or_bar(atom when atom in [:foo, :bar])

Of course nobody would write such in practice but supporting it would allow you to express patterns with guards:

def is_foo_or_bar(foo_or_bar())

Which the Elixir compiler would then rewrite internally as:

def is_foo_or_bar(atom) when atom in [:foo, :bar]

Anyway, this is very exciting and it is close to topics I am currently researching. I could expand more on both points above if you are interested. For example, if you are able to generalize defpat to records, you should also be able to generalize it for map updates, and if you define a pattern such as:

defpat foo_bar_baz %{"foo" => {bar, baz}}

And then allow someone to update the nested tuple like this:

map = %{"foo" => {1, 2}, "hello" => "world"}
foo_bar_baz(map, bar: 3)
#=> %{"foo" => {3, 2}, "hello" => "world"}

Optimizations could allow you to compile foo_bar_baz to %{map | "foo" => Map.fetch!(map, "foo") |> put_elem(0, 3)}.

Hey, good news, I’ve just released v1.0, I did a major rewrite as I really wanted (and needed) it to support guards. Wrote much a much better README guide (I guess) and also documented it more.

The only thing I removed from v0 is the ... syntax as it introduced all variables in scope and it was mostly a pain since elixir 1.5. So now you have to be explicit on what you bind.

Hope the guide explains a bit better where this library fits and how it could be used.

<3

Edit. Forgot to mention, for @josevalim, in the example using guards from the readme, expat def just expands the inner patterns and collects (anding) any guards produced by them, and finally just ands those with any from the function definition. here’s the code

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!