Expat

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

Installation

Available in Hex, the package can be installed as:

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

```elixir
def deps do
  [{:expat, "~> 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. expat helps by giving you a way to create these patterns easily.

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

The _ function is special inside expat, 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 Expat
...> expr = quote do
...>   fn a, b, c -> a + b + c end
...> end
...> case expr do
...>   expat(fn _, _, _({name, _, _}) -> _ end) -> name
...>   _ -> :dunno
...> end
:c

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

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

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

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

Note that expat 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.