protocols

# protocols [![Build Status](https://travis-ci.org/citizen428/protocols.png?branch=master)](https://travis-ci.org/citizen428/protocols)

`defprotocol` and `defimpl` macros inspired by [Elixir](http://elixir-lang.org/getting-started/protocols.html).
Under the hood this uses modules and abstract methods to make sure classes claiming to adhere to a certain protocol
actually implement it fully.

## Installation


Add this to your application's `shard.yml`:

```yaml
dependencies:
  protocols:
    github: citizen428/protocols
```

## Usage

```crystal
require "protocols"
```

### Definining protocols

`defprotocol` is used for defining new protocols (which will be created as toplevel modules).

```crystal
defprotocol Greetable do
  def greet
    "Hello from #{name}"
  end

  abstract def name
end
```

The above form allows us to specify both concrete and abstract methods for the protocol. There is
also a shortcut for protocols which only contain abstract methods:

```crystal
defprotocol Greetable, :greet, :name
```

### Implementing protocols

Given the following `Blank` protocol

```crystal
# Returns true if if data is considered blank/empty
defprotocol Blank, :blank?
```

we can define define an implementation inside a class or struct body:

```crystal
class Array
  defimpl Blank do
    def blank?
      true
    end
  end
end

nil.blank? #=> true
```

`defimpl` will `include` the protocol module, define a nested module called `"#{protocol_name}Implementation"`
and include this module too. While this may seem complicated, it makes it possible to `derive` implementations
from other classes.

### Deriving protocols

To avoid code duplication, it's possible to `derive` protocol implementations from other classes/structs:

```crystal
struct Tuple
  derive Blank, from: Array
end

Tuple.new.blank? #=> true
```

## Todo

* [ ] Write specs

## Contributing

1. Fork it ( https://github.com/citizen428/protocols/fork )
2. Create your feature branch (`git checkout -b my-new-feature`)
3. Commit your changes (`git commit -am 'Add some feature'`)
4. Push to the branch (`git push origin my-new-feature`)
5. Create a new Pull Request

## Contributors

- [citizen428](https://github.com/citizen428) Michael Kohl - creator, maintainer