Embedded Domain Specific Languages (eDSL) in Scala

An informal uninformed introduction

Today's menu

Domain Specific Languages

    Uses

External DSLs

    Scala support (parser combinators)

Internal/embedded DSLs

    Sample(s)

    Scala support

    Let's try yo build one?

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Domain Specific Languages

What are they?

 "A Domain-Specific Language is a programming language that mimics the terms, idioms, and expressions used among experts in the targeted domain" Programming Scala - A. Payne, D. Wampler

Why?

• communication
• modeling
• efficiency/quality

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Two types

•     external
•     internal/embedded

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Domain Specific Languages

Advantages

• High level of abstraction
• Self-documenting code
• Validation with domain

Disadvantages

• Cost of learning vs applicability
• Have to build/maintain it
• Proliferation! 

External DSL

• More flexible
• In Scala: parser combinators library
• Still you have to write the rest
• Example: JSON parsing (Programming in Scala)
• Express the syntax of the language close to EBNF

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  val ID = """[a-zA-Z]([a-zA-Z0-9]|_[a-zA-Z0-9])*"""r�  val NUM = """[1-9][0-9]*"""r�  def program = clazz*�  def classPrefix = "class" ~ ID ~ "(" ~ formals ~ ")"�  def classExt = "extends" ~ ID ~ "(" ~ actuals ~ ")"�  def clazz = classPrefix ~ opt(classExt) ~ "{" ~ (member*) ~ "}"�  def formals = repsep(ID ~ ":" ~ ID, ",")�  def actuals = expr*�  def member = (�      "val" ~ ID ~ ":" ~ ID ~ "=" ~ expr�    | "var" ~ ID ~ ":" ~ ID ~ "=" ~ expr�    | "def" ~ ID ~ "(" ~ formals ~ ")" ~ ":" ~ ID ~ "=" ~ expr�    | "def" ~ ID ~ ":" ~ ID ~ "=" ~ expr�    | "type" ~ ID ~ "=" ~ ID�  )

embedded DSLs

• Uses the language as a host
• Constrained by it
• Simpler to create
• Closely related to Fluent interfaces and DDD
• "Just a glorified API"
• The IDE can help you 
• syntax highlighting
• code completion

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Some (Scala) DSL examples

Tools (embedded DSL)

• SBT
• Specs (BDD framework)

EDF Trading (external DSL)

• DSL for derivatives trading and pricing

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Some (Scala) eDSL examples

def expr(a:Boolean,b:Boolean,c:Boolean,d:Boolean)=�        ( (a Λ b) V ¬(a Λ c) ) Λ ( (c V d) Λ ¬(b) )

...

val payrollCalculator = rules { employee =>

employee salary_for 2.weeks minus_deductions_for { gross =>

federalIncomeTax is (25. percent_of gross)�stateIncomeTax is (5. percent_of gross)�insurancePremiums are (500. in gross.currency)�retirementFundContributions are (10. percent_of gross)

}

}

...�  // use premium pricing strategy for order�  new Order to buy(100 sharesOf "IBM")�            maxUnitPrice 300�            using premiumPricing,

embedded DSLs: Support in Scala

Scala building blocks for embedded DSLs:

• Implicit conversions
• Optional dots, semi-colons and parentheses
• Higher order functions
• Symbols as methods
• Currying
• Implicit parameters

(and more)

    (... thinking about it... most of Scala  )

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Interesting links

http://debasishg.blogspot.com/2008/05/designing-internal-dsls-in-scala.html

 (he's working on a book all bout DSLs)

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http://programming-scala.labs.oreilly.com/ch11.html

Show me the code!!

Let's keep in touch!

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    Twitter: @gclaramunt

    Blog: http://gabrielsw.blogspot.com