README.md
# Docile
[](https://rubygems.org/gems/docile)
[](https://rubygems.org/gems/docile)
[](https://gitter.im/ms-ati/docile?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
[](http://rubydoc.info/github/ms-ati/docile)
[](https://github.com/ms-ati/docile/actions/workflows/main.yml)
[](https://codecov.io/github/ms-ati/docile)
[](https://codeclimate.com/github/ms-ati/docile/maintainability)
Ruby makes it possible to create very expressive **Domain Specific
Languages**, or **DSL**'s for short. However, it requires some deep knowledge and
somewhat hairy meta-programming to get the interface just right.
"Docile" means *Ready to accept control or instruction; submissive* [[1]]
Instead of each Ruby project reinventing this wheel, let's make our Ruby DSL
coding a bit more docile...
[1]: http://www.google.com/search?q=docile+definition "Google"
## Usage
### Basic: Ruby [Array](http://ruby-doc.org/core-3.0.0/Array.html) as DSL
Let's say that we want to make a DSL for modifying Array objects.
Wouldn't it be great if we could just treat the methods of Array as a DSL?
```ruby
with_array([]) do
push 1
push 2
pop
push 3
end
#=> [1, 3]
```
No problem, just define the method `with_array` like this:
```ruby
def with_array(arr=[], &block)
Docile.dsl_eval(arr, &block)
end
```
Easy!
### Next step: Allow helper methods to call DSL methods
What if, in our use of the methods of Array as a DSL, we want to extract
helper methods which in turn call DSL methods?
```ruby
def pop_sum_and_push(n)
sum = 0
n.times { sum += pop }
push sum
end
Docile.dsl_eval([]) do
push 5
push 6
pop_sum_and_push(2)
end
#=> [11]
```
Without Docile, you may find this sort of code extraction to be more
challenging.
### Wait! Can't I do that with just `instance_eval` or `instance_exec`?
Good question!
In short: **No**.
Not if you want the code in the block to be able to refer to anything
the block would normally have access to from the surrounding context.
Let's be very specific. Docile internally uses `instance_exec` (see [execution.rb#26](lib/docile/execution.rb#L26)), adding a small layer to support referencing *local variables*, *instance variables*, and *methods* from the _block's context_ **or** the target _object's context_, interchangeably. This is "**the hard part**", where most folks making a DSL in Ruby throw up their hands.
For example:
```ruby
class ContextOfBlock
def example_of_contexts
@block_instance_var = 1
block_local_var = 2
with_array do
push @block_instance_var
push block_local_var
pop
push block_sees_this_method
end
end
def block_sees_this_method
3
end
def with_array(&block)
{
docile: Docile.dsl_eval([], &block),
instance_eval: ([].instance_eval(&block) rescue $!),
instance_exec: ([].instance_exec(&block) rescue $!)
}
end
end
ContextOfBlock.new.example_of_contexts
#=> {
:docile=>[1, 3],
:instance_eval=>#<NameError: undefined local variable or method `block_sees_this_method' for [nil]:Array>,
:instance_exec=>#<NameError: undefined local variable or method `block_sees_this_method' for [nil]:Array>
}
```
As you can see, it won't be possible to call methods or access instance variables defined in the block's context using just the raw `instance_eval` or `instance_exec` methods. And in fact, Docile goes further, making it easy to maintain this support even in multi-layered DSLs.
### Build a Pizza
Mutating (changing) an Array instance is fine, but what usually makes a good DSL is a [Builder Pattern][2].
For example, let's say you want a DSL to specify how you want to build a Pizza:
```ruby
@sauce_level = :extra
pizza do
cheese
pepperoni
sauce @sauce_level
end
#=> #<Pizza:0x00001009dc398 @cheese=true, @pepperoni=true, @bacon=false, @sauce=:extra>
```
And let's say we have a PizzaBuilder, which builds a Pizza like this:
```ruby
Pizza = Struct.new(:cheese, :pepperoni, :bacon, :sauce)
class PizzaBuilder
def cheese(v=true); @cheese = v; self; end
def pepperoni(v=true); @pepperoni = v; self; end
def bacon(v=true); @bacon = v; self; end
def sauce(v=nil); @sauce = v; self; end
def build
Pizza.new(!!@cheese, !!@pepperoni, !!@bacon, @sauce)
end
end
PizzaBuilder.new.cheese.pepperoni.sauce(:extra).build
#=> #<Pizza:0x00001009dc398 @cheese=true, @pepperoni=true, @bacon=false, @sauce=:extra>
```
Then implement your DSL like this:
```ruby
def pizza(&block)
Docile.dsl_eval(PizzaBuilder.new, &block).build
end
```
It's just that easy!
[2]: http://stackoverflow.com/questions/328496/when-would-you-use-the-builder-pattern "Builder Pattern"
### Multi-level and Recursive DSLs
Docile is a very easy way to write a multi-level DSL in Ruby, even for
a [recursive data structure such as a tree][4]:
```ruby
Person = Struct.new(:name, :mother, :father)
person {
name 'John Smith'
mother {
name 'Mary Smith'
}
father {
name 'Tom Smith'
mother {
name 'Jane Smith'
}
}
}
#=> #<struct Person name="John Smith",
# mother=#<struct Person name="Mary Smith", mother=nil, father=nil>,
# father=#<struct Person name="Tom Smith",
# mother=#<struct Person name="Jane Smith", mother=nil, father=nil>,
# father=nil>>
```
See the full [person tree example][4] for details.
[4]: https://gist.github.com/ms-ati/2bb17bdf10a430faba98
### Block parameters
Parameters can be passed to the DSL block.
Supposing you want to make some sort of cheap [Sinatra][3] knockoff:
```ruby
@last_request = nil
respond '/path' do |request|
puts "Request received: #{request}"
@last_request = request
end
def ride bike
# Play with your new bike
end
respond '/new_bike' do |bike|
ride(bike)
end
```
You'd put together a dispatcher something like this:
```ruby
require 'singleton'
class DispatchScope
def a_method_you_can_call_from_inside_the_block
:useful_huh?
end
end
class MessageDispatch
include Singleton
def initialize
@responders = {}
end
def add_responder path, &block
@responders[path] = block
end
def dispatch path, request
Docile.dsl_eval(DispatchScope.new, request, &@responders[path])
end
end
def respond path, &handler
MessageDispatch.instance.add_responder path, handler
end
def send_request path, request
MessageDispatch.instance.dispatch path, request
end
```
[3]: http://www.sinatrarb.com "Sinatra"
### Functional-Style Immutable DSL Objects
Sometimes, you want to use an object as a DSL, but it doesn't quite fit the
[imperative](http://en.wikipedia.org/wiki/Imperative_programming) pattern shown
above.
Instead of methods like
[Array#push](http://www.ruby-doc.org/core-3.0.0/Array.html#method-i-push), which
modifies the object at hand, it has methods like
[String#reverse](http://www.ruby-doc.org/core-3.0.0/String.html#method-i-reverse),
which returns a new object without touching the original. Perhaps it's even
[frozen](http://www.ruby-doc.org/core-3.0.0/Object.html#method-i-freeze) in
order to enforce [immutability](http://en.wikipedia.org/wiki/Immutable_object).
Wouldn't it be great if we could just treat these methods as a DSL as well?
```ruby
s = "I'm immutable!".freeze
with_immutable_string(s) do
reverse
upcase
end
#=> "!ELBATUMMI M'I"
s
#=> "I'm immutable!"
```
No problem, just define the method `with_immutable_string` like this:
```ruby
def with_immutable_string(str="", &block)
Docile.dsl_eval_immutable(str, &block)
end
```
All set!
### Accessing the block's return value
Sometimes you might want to access the return value of your provided block,
as opposed to the DSL object itself. In these cases, use
`dsl_eval_with_block_return`. It behaves exactly like `dsl_eval`, but returns
the output from executing the block, rather than the DSL object.
```ruby
arr = []
with_array(arr) do
push "a"
push "b"
push "c"
length
end
#=> 3
arr
#=> ["a", "b", "c"]
```
```ruby
def with_array(arr=[], &block)
Docile.dsl_eval_with_block_return(arr, &block)
end
```
## Features
1. Method lookup falls back from the DSL object to the block's context
2. Local variable lookup falls back from the DSL object to the block's
context
3. Instance variables are from the block's context only
4. Nested DSL evaluation, correctly chaining method and variable handling
from the inner to the outer DSL scopes
5. Alternatives for both imperative and functional styles of DSL objects
## Installation
``` bash
$ gem install docile
```
## Links
* [Source](https://github.com/ms-ati/docile)
* [Documentation](http://rubydoc.info/gems/docile)
* [Bug Tracker](https://github.com/ms-ati/docile/issues)
## Status
Works on [all currently supported ruby versions](https://github.com/ms-ati/docile/blob/master/.github/workflows/main.yml),
or so [Github Actions](https://github.com/ms-ati/docile/actions)
tells us.
Used by some pretty cool gems to implement their DSLs, notably including
[SimpleCov](https://github.com/colszowka/simplecov). Keep an eye out for new
gems using Docile at the
[Ruby Toolbox](https://www.ruby-toolbox.com/projects/docile).
## Release Policy
Docile releases follow
[Semantic Versioning 2.0.0](https://semver.org/spec/v2.0.0.html).
## Note on Patches/Pull Requests
* Fork the project.
* Setup your development environment with:
`gem install bundler; bundle install`
* Make your feature addition or bug fix.
* Add tests for it. This is important so I don't break it in a future version
unintentionally.
* Commit, do not mess with rakefile, version, or history.
(if you want to have your own version, that is fine but bump version in a
commit by itself I can ignore when I pull)
* Send me a pull request. Bonus points for topic branches.
## Releasing
To make a new release of `Docile` to
[RubyGems](https://rubygems.org/gems/docile), first install the release
dependencies (e.g. `rake`) as follows:
```shell
bundle config set --local with 'release'
bundle install
```
Then carry out these steps:
1. Update `HISTORY.md`:
- Add an entry for the upcoming version _x.y.z_
- Move content from _Unreleased_ to the upcoming version _x.y.z_
- Commit with title `Update HISTORY.md for x.y.z`
2. Update `lib/docile/version.rb`
- Replace with upcoming version _x.y.z_
- Commit with title `Bump version to x.y.z`
3. `bundle exec rake release`
## Copyright & License
Copyright (c) 2012-2022 Marc Siegel.
Licensed under the [MIT License](http://choosealicense.com/licenses/mit/),
see [LICENSE](LICENSE) for details.