Code generation with Scala

Question

When using the SBT toolchain in Scala, is it possible to write a task that will read a special part of the project's source to generate scala-code at compile time.

Any ideas or even articles/tutorials on this? I am looking for something rather similar to Template Haskell.

Answer 1

treehugger.scala is a library designed for code generation.

import treehugger.forest._
import definitions._
import treehuggerDSL._

val tree: Tree = Predef_println APPLY LIT("Hello, world!")

println(tree)
println(treeToString(tree))

The above code prints two lines:

Apply(Ident(println),List(Literal(Constant(Hello, world!))))
println("Hello, world!")

treehugger does generate an AST, but non-compliant to scalac's AST.

Answer 2

Scala 2.10 has experimental support for macros which alike sophisticated compile-time code generation. See here for more detail.

There are some fun examples on Jason Zaugg's macrocosm git repository, and the SLICK library which is an evolution of the ScalaQuery SQL DSL enabling type-safe database (and collection) queries to be expressed in a LINQ-like way.

And this example, from the expecty assertion library:

import org.expecty.Expecty

case class Person(name: String = "Fred", age: Int = 42) {
  def say(words: String*) = words.mkString(" ")
}

val person = Person()
val expect = new Expecty()

...
val word1 = "ping"
val word2 = "pong"

expect {
  person.say(word1, word2) == "pong pong"
}

Yielding:

java.lang.AssertionError:

person.say(word1, word2) == "pong pong"
|      |   |      |      |
|      |   ping   pong   false
|      ping pong
Person(Fred,42)