Domain specific languages

Domain Specific Languages
Dror Helper

About.Me
Software developer
Blogger (http://blog.drorhelper.com)
Language geek

What do they mean?
Bug
Feature
Release Candidate
Show stopper
“The next release candidate would contain features X, Y & Z and no more than 5 high priority Bugsandno show stoppers”

Agenda
Why should I use DSLs
Fluent interface

created specifically to solve problems in a particular domain
Layer of abstraction on top of an existing domain model
Can be used by domain experts

DSL classification
External vs. Internal
Textual vs. Graphic

Graphical DSLs

External DSLs
SELECTBook.title, count(*) ASAuthors FROMBook JOINBook_authorONBook.isbn = Book_author.isbnGROUPBYBook.title;
<([A-Z][A-Z0-9]*)b[^>]*>(.*?)</1>
buyIBM @ 100 forNOMURA sellGOOGLE @ limitprice= 70 for CHASE

Internal DSLs
Built on top of an existing language
Use languages like Ruby, Python, BOO

Fluent interfaces
coined by Eric Evans and Martin Fowler
An OOP API that aims to provide readable code
Use method chaining an pass context via return value
TimeIntervalmeetingTime = fiveOClock.until(sixOClock);
DateTime.Now.AddDays(7);  DateTime.Now.AddWeeks(1);

Fluent Interfaces - Examples

Fluent Interface design
Hide your working.
Keep your state to yourself.
Think really hard about names.
Take advantage of your implementation language.
If you have them, blocks are you friends.
Test first design can be a useful way of exploring what your interface should be.
Reasonable defaults.

Tips & Tricks
Use Interfaces
Return different interface to change functionality
Generics are handy if user need to specify type
Lambda and delegates for custom functionality

Books
DSLs in Action
DSLs in Boo

The shopping cart
A Domain Specific exercise

The Story
You need to create a simple rule engine that would enable developers to add custom rules to the company’s Shopping cart application.

The engine
Three customer types: regular, preferred & VIP
Customer the following fields:
name
address
date of birth
The system tracks the customer’s purchases (date, amount)
Products can be on Sale (discount)
Discount changes according to rules
Each rule must have a customer type

Usage examples
A VIP customer get 15% discount on all purchases
Buying product A would give you 10% discount on product B
Preferred and VIP customers can buy 5 + 1 of a specific product
All customers can get 20% off purchase using specific coupon
If amount of purchase > 100$ customer is entitled for 10% discount.
If customer have purchased > 100$ the last three months upgrade him to VIP
On the customer’s birthday he’s entitled to 50% discount
All customers living in New York can get up to 25% off all purchases

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Domain specific languages

by Dror Helper on Mar 23, 2011

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