Domain Specific Languages In Scala Duse3

Domain Specific Languages #DUSE-3

Peter Maas ~ Developer at Ebay Classifieds, working at the Marktplaats.nl migration. Likes programming languages. whoami

Agenda
What is a DSL?
What makes Scala a good language for DSLs
Internal DSL
External DSL
ACTUAL CODE!

Questions? Feel free to interrupt!

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.

A language which feels natural to a specialist in the targeted domain

Internal / External
Internal DSLs are built using the language itself.
Buildr
Gradle
Routing configuration in Apache Camel
External DSLs are 'mini' programming languages with their own compiler/interpreter
SQL
Velocity

Why is Scala good at that
Concise and tolerant syntax
Function objects (closures, currying etc)
Default arguments
Operator overloading
Implicit conversions
Built-in support for building parsers
Due to it's functional nature it is good at handling syntax trees.

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)

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"

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" )

Let's build a DSL we all know SQL http://github.com/p3t0r/scala-sql-dsl

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 ;-)

Syntax Tree

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

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: _*) }

That looks like a lot of code :( The entire builder is only 54 lines, including whitespace and comments! >> CONTINUE IN IDEA <<

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 */ }

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Domain Specific Languages In Scala Duse3

by Peter Maas

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