vic/macpato

---

vic/macpato.json

{
  "defaultBranch": "master",
  "description": "Simple pattern matching on Elixir quoted expressions.",
  "fullName": "vic/macpato",
  "homepage": "https://hex.pm/packages/macpato",
  "language": "Elixir",
  "name": "macpato",
  "pushedAt": "2017-01-11T16:47:53Z",
  "stargazersCount": 6,
  "updatedAt": "2020-08-25T15:09:03Z",
  "url": "https://github.com/vic/macpato"
}

Macpato

Macpato is a macro for very simple pattern matching on quoted elixir expressions.

Installation

Available in Hex, the package can be installed as:

1. Add macpato to your list of dependencies in mix.exs:

```elixir

def deps do
  [{:macpato, "~> 0.1.2"}]
end
```

Usage

A typical pattern match on a quoted expression looks like this:

{:foo, _ctx, _args} = quote(do: foo)

However for complex expression, the pattern to match the AST can get way longer. macpato helps by giving you a way to create these patterns easily.

iex> import Macpato
...> expr = quote do
...>   fn a, b, c -> a + b + c end
...> end
...> case expr do
...>   macpato(fn _, b, _ -> _ end) -> :b_is_second_arg
...>   _ -> :dunno
...> end
:b_is_second_arg

The _ function is special inside macpato, and when given an argument it will just place whatever you give it into the pattern.

This way you can for example, assign part of the match into a variable in this case to get the name of the third argument bellow:

iex> import Macpato
...> expr = quote do
...>   fn a, b, c -> a + b + c end
...> end
...> case expr do
...>   macpato(fn _, _, _({name, _, _}) -> _ end) -> name
...>   _ -> :dunno
...> end
:c

Or use a pinned value, for example to check we are adding the number 22.

iex> import Macpato
...> expr = quote do
...>   fn a -> a + 22 end
...> end
...> x = 22
...> case expr do
...>   macpato(fn _ -> _ + _(^x) end) -> :good
...>   _ -> :dunno
...> end
:good

You can capture arrays from the AST with _(@) for example:

iex> import Macpato
...> expr = quote do
...>   fn a, b, c -> x end
...> end
...> case expr do
...>   macpato(fn _(@args) -> _ end) -> length(args)
...> end
3

Note that macpato can be used on any place where you can have a pattern in Elixir, like macro definition arguments, cases, with expressions, etc.

See the tests for more examples.