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AestasIT - Internal DSLs in Scala


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  • 1. Internal DSLs in ScalaAndrey AdamovichAestas/IT
  • 2. What’s in thispresentation? Definition and usage of DSLs Overview of Scala features that can be used to create internal DSL Examples of Scala-based DSLs Some links and reading material Questions
  • 4. What is DSL?Domain-Specific Language” A Domain-Specific Language is a programming language or executable specification language that offers, through appropriate notations and abstractions, expressive power focused on, and usually restricted to, a particular problem domain.” The opposite is: • a general-purpose programming language, such as C, Java or Python, • or a general-purpose modeling language such as the Unified Modeling Language (UML).
  • 5. Examples of DSL”    Examples of domain-specific languages include: HTML, Logo for children,  Verilog and VHDL hardware description languages,  Mata for matrix programming,  Mathematica and Maxima for symbolic mathematics,  spreadsheet formulas and macros,  SQL for relational database queries,  YACC grammars for creating parsers,  regular expressions for specifying lexers,  the Generic Eclipse Modeling System for creating diagramming languages,  Csound for sound and music synthesis,  and the input languages of GraphViz and GrGen, software packages used for graph layout and graph rewriting.
  • 6. Goals of DSL”  Use a more expressive language than a general-purpose one  Share a common metaphor of understanding between developers and subject matter experts  Have domain experts help with the design of the business logic of an application  Avoid cluttering business code with too much boilerplate technical code thanks to a clean separation  Let business rules have their own lifecycle Guillaume Laforge Groovy Project Manager
  • 7. What is internal DSL?” Internal DSLs are particular ways of using a host language to give the host language the feel of a particular language Martin Fowler
  • 8. Library vs. DSL Is there any difference between a well-structured library or API and an internal DSL? Not much, except for the following: • Internal DSL does not look like code in host language • It’s more readable in general and is much closer to natural language
  • 9. Fluent API” In software engineering, a fluent interface (as first coined by Eric Evans and Martin Fowler) is an implementation of an object oriented API that aims to provide for more readable code.” A fluent interface is normally implemented by using method chaining to relay the instruction context of a subsequent call (but a fluent interface entails more than just method chaining).
  • 10. Fluent API examplesMockito
  • 11. SCALA
  • 12. Features of Scala Lightweight syntax Combines functional and object- oriented aproaches Advanced type system: everything has a type Strong type inference Performance comparable to Java Fully interoperable with Java
  • 13. What makes internal DSLpossible in Scala? ”Dot-free”, infix and postfix operator notation ”Bracket-free” function calls (Almost) any character can be used in method names Implicit conversions Advanced type system By-name parameters and currying
  • 14. Lightweight syntaxval numbers = List(1, 2, 3)numbers map { x => x + 1 }numbers sortWith { (x, y) => x > y }numbers map { _ + 1 } sortWith { _ > _ }numbers size
  • 15. Implicit conversionsMap( "first" -> "Test", "second" -> "Code" )/* Defines a new method sort for array objects */object implicits extends Application { implicit def arrayWrapper[A : ClassManifest](x: Array[A]) = new { def sort(p: (A, A) => Boolean) = { util.Sorting.stableSort(x, p); x } } val x = Array(2, 3, 1, 4) println("x = "+ x.sort((x: Int, y: Int) => x < y))}
  • 16. By-name parametersdebug("This" + " is" + " very" + " costly!")def debug(msg: => String): Unit = if (isDebugEnabled()) println(msg)spawn(println("I run in different thread!"))def spawn(p: => Unit) = { val t = new Thread() { override def run() = p } t.start() }
  • 17. Curryng I” In mathematics and computer science, currying is the technique of transforming a function that takes multiple arguments (or an n-tuple of arguments) in such a way that it can be called as a chain of functions each with a single argument (partial application).
  • 18. Curryng IIusing(new BufferedReader(new FileReader("file"))) { r => var count = 0 while (r.readLine != null) count += 1 println(count)}def using[T <: { def close() }] (resource: T) (block: T => Unit){ try { block(resource) } finally { if (resource != null) resource.close() }}
  • 19. Advanced type system
  • 21. BaysickScalaBasicRunner.scala:object ScalaBasicRunner extends Baysick with Application { 10 PRINT "Scala" 20 LET (number := 1) 30 IF number > 0 THEN 50 40 PRINT "Java" 50 PRINT "rulez!" 60 END RUN}
  • 22. Time DSLimport + 2.monthsDateTime.nextMonth < + to DateTime.tomorrow( to DateTime.nextSecond).millis2.hours + 45.minutes + 10.seconds(2.hours + 45.minutes + 10.seconds).millis2.months + 3.days
  • 23. Spring Integration DSLval messageFlow = filter using { payload: String => payload == "World" } --> transform using { payload: String => "Hello " + payload } --> handle using { payload: String => println(payload) }messageFlow.send("World")
  • 25. BooksProgramming in Scala, Second Edition Martin Odersky, Lex Spoon, and Bill Venners DSLs in Action Debasish Ghosh
  • 26. Links time ntroducing-spring-integration-scala-dsl/ a-scala-dsl-implementing-basic/