How to make DSL

DSL Powered by Rabbit 2.1.2
DSL
Yukio Goto
sg.rb
2014/04/29

Who am I ?
Yukio Goto
favorite
Ruby, emacs, zsh, tennis✓
✓
work
Rakuten Asia Pte. Ltd.✓
As senior application engineer✓
✓
✓

IDs
github
https://github.com/byplayer/
twitter
@byplayer

Today's target
use DSL
-*-*-*-*-*-*-
making DSL

DSL
What is DSL ?

DSL
DSL =
Domain Speciﬁc
Language

external DSL
SQL✓
SELECT id, user_name FROM users

external DSL
conﬁguration ﬁles✓
http {
passenger_root /usr/local/rvm/gems/ruby-2.1.0/gems/passenger-4.0.29;
passenger_ruby /usr/local/rvm/wrappers/ruby-2.1.0/ruby;
include mime.types;
default_type application/octet-stream;
# ...
}

internal DSL
Rails✓
class User < ActiveRecord::Base
validates :name, presence: true
end

BTW
By the way

Did you make DSL ?
Did you make
DSL ?

use only ?
Do you think
you can only
use DSL?

No
NO !!!

You
You

can
can

make
make

your own DSL
your own
DSL

AND
AND

Ruby
Ruby
is

easiest language
one of the most
easiest
language

making DSL
making DSL

How to make DSL
How to make
DSL

before that
Before that

Beneﬁt of making DSL
DSL makes your Application
easy to write✓
easy to read✓
easy to use✓
✓

theme
Conﬁguration ﬁle using DSL

conﬁg version 1
# singapore-ruby-group.conf
meetup do |conf|
conf.group = 'Singapore-Ruby-Group'
conf.organizer = 'Winston Teo'
conf.sponsors = ['Engine Yard',
'Silicon Straits',
'Plug-In@Blk71']
end

how to use
mt = Meetup.load('singapore-ruby-group.conf')
puts mt.group
# => Singapore-Ruby-Group
puts mt.organizer
# => Winston Teo
puts mt.sponsors.join(', ')
# => Engine Yard, Silicon Straits, Plug-In@Blk71

implementation
# Sample class to load configuration
class Meetup
attr_accessor :group, :organizer, :sponsors
def self.load(path)
fail "config file not found: #{path}" unless File.exist?(path)
mt = Meetup.new
File.open(path, 'r') do |file|
mt.instance_eval(File.read(path), path)
end
mt
end
private
def meetup
yield self
end
end

key point
def self.load
# ...
end
# ...
end
def meetup
yield self
end

ﬁrst point
pass string to 'instance_eval'
def self.load
# ...
end
# ...
end

instance_eval
"instance_eval"

instance_eval
interpretes
String

instance_eval
as Ruby code

instance_eval
using Object
context

instance_eval
In this case,

instance_eval
conﬁguration
ﬁle is

instance_eval
interpreted as

instance_eval
Meetup class
source code

expand instance_eval virtually
def self.load
# File.open(path, 'r') do |file|
# mt.instance_eval(File.read(path), path)
# end
meetup do |conf|
'Silicon Straits',
'Plug-In@Blk71']
end
end

easy
quite easy

but
BUT

Typing conﬁg
Typing 'conf.'

is is
is not

sexy
sexy

Is it sexy ?
meetup do |conf|
'Silicon Straits',
'Plug-In@Blk71']
end

ideal conﬁguration
# singapore-ruby-group.conf
meetup {
group 'Singapore-Ruby-Group'
organizer 'Winston Teo'
sponsors ['Engine Yard',
'Silicon Straits',
'Plug-In@Blk71']
}

readable
readable✓
DRY✓

Let's use
Let's use

Ruby's black magic
Ruby's black
magic

parsing
parsing it

load
class Meetup
def self.load(path)
mt = new
loader = MeetupLoader.new(mt)
loader.instance_eval(file.read, path)
end
mt
end
end

make support class
make support class
MeetupLoader

make support class
deﬁne setter
method

make support class
not use '='

make support class
and

make support class
load function
(meetup) only

make support class (setter)
class MeetupLoader
def initialize(mt)
@meetup = mt
end
# ...
def group(g)
@meetup.group = g
end
def organizer(o)
@meetup.organizer = o
end
# ...
end

instance_eval again
class MeetupLoader
# ...
def meetup(&block)
instance_eval(&block)
end
# ...
end

The point of black magic
instance_eval
again

instance_eval for block
instance_eval
with block

changes
changes

default receiver
default
receiver

in the block
in the block

original code
class Meetup
def self.load(path)
mt = new
loader.instance_eval(file.read, path)
end
mt
end
end

expand instance_eval virtually 1
def self.load(path)
mt = new
# loader.instance_eval(file.read, path)
loader.meetup {
group 'Singapore-Ruby-Group'
organizer 'Winston Teo'
# ...
}
end
#...

instance_eval with block
class MeetupLoader
# ...
def meetup(&block)
instance_eval(&block)
end
# ...
end

expand instance_eval virtually 2
def meetup(&block)
# instance_eval(&block)
#{
self.group 'Singapore-Ruby-Group'
self.organizer 'Winston Teo'
# ...
# }
end

The trick
The trick

of
of

is
is

instance_eval with block
instance_eval
with block

Today
Today,

I
I

don't
don't

talk about
talk about

caution points
caution points

caution points
security✓
handle method_missing✓
handle syntax error✓

source code
https://github.com/byplayer/meetup_conﬁg✓
https://github.com/byplayer/
meetup_conﬁg_ex
✓

take a way
You can make DSL✓
instance_eval is interesting✓

Hiring
Rakuten Asia Pte. Ltd.
wants to hire
Server side Application
Engineer (Ruby, java)
✓
iOS, Android Application
Engineer
✓
✓

Any question ?
Any question ?

Thank you
Thank you
for listening

How to make DSL

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