Domain Specific Languages In Scala Duse3
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The document discusses Domain-Specific Languages (DSLs), which are tailored programming languages focused on particular problem domains. It highlights Scala's strengths for creating DSLs, including concise syntax, functional capabilities, and built-in parser support. Examples of internal and external DSLs in Scala are provided, along with code snippets demonstrating how to implement such languages effectively.
Domain Specific Languages In Scala Duse3
- 1. Domain Specific Languages #DUSE-3
- 2. Peter Maas ~ Developer at Ebay Classifieds, working at the Marktplaats.nl migration. Likes programming languages. whoami
- 3. Agenda What is a DSL?
- 4. What makes Scala a good language for DSLs
- 5. Internal DSL
- 6. External DSL ACTUAL CODE!
- 7. Questions? Feel free to interrupt!
- 8. So, what is a DSL a D omain- S pecific L anguage (DSL) is a programming language or specification language dedicated to a particular problem domain, a particular problem representation technique, and/or a particular solution technique.
- 9. A language which feels natural to a specialist in the targeted domain
- 10. Internal / External Internal DSLs are built using the language itself. Buildr
- 11. Gradle
- 12. Routing configuration in Apache Camel External DSLs are 'mini' programming languages with their own compiler/interpreter SQL
- 13. Velocity
- 14. Why is Scala good at that Concise and tolerant syntax
- 15. Function objects (closures, currying etc)
- 19. Built-in support for building parsers
- 20. Due to it's functional nature it is good at handling syntax trees.
- 21. Concise? val toGreet = " Marktplaats developers" "Hello" . + ( toGreet ) "Hello" + toGreet "Hello" . concat ( toGreet ) "Hello" concat toGreet // methods are objects... val greetMethod = "hello %s".format(_:String) greetMethod(toGreet)
- 22. Implicit conversions class StringReducer(s:String) { def acronym = s.filter(_.isUpper) } implicit def string2StringReducer(s:String) = new StringReducer(s) "Sun Certified Java Programmer".acronym == "SCJP"
- 23. Or... case class Route(source:String, target:String) class RouteBuilder(s:String) { def --> (target:String) = Route(s, target) } implicit def string2Route(s:String) = new RouteBuilder(s) "a" --> "b" == Route( "a" , "b" )
- 24. Let's build a DSL we all know SQL http://github.com/p3t0r/scala-sql-dsl
- 25. Pure Scala // Simple query with sorting SQL select "*" from ("user") order Asc("name") // More complex query with and/or including precedence SQL select "*" from ("user") where (("name","peter") and (("active", true) or ("role", "admin"))) Overriding the '=' operator was a no-go ;-)
- 26. Syntax Tree
- 27. case class Where( val clauses: Clause*) trait Clause { def and (otherField: Clause): Clause = And(this, otherField) def or (otherField: Clause): Clause = Or(this, otherField) } case class StringEquals(val f: String, val value: String) extends Clause case class NumberEquals(val f: String, val value: Number) extends Clause case class BooleanEquals(val f: String, val value: Boolean) extends Clause case class In(val field: String, val values: String*) extends Clause case class And(val lClause:Clause, val rClause:Clause) extends Clause case class Or(val lClause:Clause, val rClause:Clause) extends Clause AST for the Where clause
- 28. Some Implicits & Helpers object QueryBuilder { implicit def tuple2field(t: (String, String)): StringEquals = StringEquals(t._1, t._2) implicit def tuple2field(t: (String, Int)): NumberEquals = NumberEquals(t._1, t._2) implicit def tuple2field(t: (String, Boolean)): BooleanEquals = BooleanEquals(t._1, t._2) implicit def from2query(f: From): Query = Query(f.operation.get, f, Option(Where())) /** entrypoint for starting a select query */ def select(fields:String*) = Select(fields:_*) def select(symbol: Symbol): Select = symbol match { case 'all => select("*") case _ => throw new RuntimeException("Only 'all allowed as symbol") } def in(field: String, values: String*) = In(field, values: _*) }
- 29. That looks like a lot of code :( The entire builder is only 54 lines, including whitespace and comments! >> CONTINUE IN IDEA <<
- 30. And external? class SQLParser extends JavaTokenParsers { def query:Parser[Query] = operation ~ from ~ opt(where) ~ opt(order) ^^ { case operation ~ from ~ where ~ order => Query(operation, from, where, order) } def operation:Parser[Operation] = { ("select" | "update" | "delete") ~ repsep(ident, ",") ^^ { case "select" ~ f => Select(f:_*) case _ => throw new IllegalArgumentException("Operation not implemented") } } def from:Parser[From] = "from" ~> ident ^^ (From(_)) def where:Parser[Where] = "where" ~> rep(clause) ^^ (Where(_:_*)) /* more in idea */ }