#渋谷java

1. Macro in Scala
Naoki Takezoe
@takezoen
BizReach, Inc
1 Oct. 2016 #渋谷java 17th

2. What's Macro?

3. C
Expand special directive in source code
before compilation by pre-processor
#define square(x) ((x) * (x))
printf("i * i = %dn", i, square(i));
#define DEBUG
#ifdef DEBUG
printf("debug: i = %dn", i)
#endif

4. Lisp (also Scala or Rust)
Generate AST in compile-time
(defmacro when (test &body body)
`(if ,test (progn ,@body)))
(progn
(when (= 1 1)
(print "Hello,")
(print "World!!")))
(progn
(if (= 1 1)
(progn
(print "Hello,")
(print "World!!"))))
Expand by Macro

5. Example in Scala
def assert(cond: Boolean, msg: String): Unit
= macro assertImpl
def assertImpl(c: Context)
(cond: c.Expr[Boolean], msg: c.Expr[String]) = {
import c.universe._
q"if(!$cond){ throw new AssertionError($msg) }"
}

6. Example in Scala
assert(i == 1, "i does not 1")
if(i == 1){
throw new AssertionError("i does not 1")
}
Expand by Macro
Note: Macro have to be compiled before main compilation

7. Use cases

8. Use cases
● Validation
● Type generation
● DSL
● Optimization
without runtime overhead

9. Quill
● Database access library for Scala
● Do in compile-time by macro
○ Translate typesafe DSL to SQL
○ SQL validation against real database
val q = quote {
query[Person].filter(p => p.age > 18)
}
// SELECT p.id, p.name, p.age FROM Person p WHERE p.age > 18
http://getquill.io/

10. Macro in Scala

11. Macro type
● def macro
○ callable as same as normal function
● Macro annotation
○ triggered by annotation
○ requires Macro Paradise plug-in
@main object Test {
println("hello world")
}
object Test {
def main(args: Array[String]): Unit = {
println("hello world")
}
}
Expand by Macro annotation

12. How to construct AST?
● AST model
● reify
● Quasiquotes

13. AST model
def compileTimeImpl(c: Context)() = {
Literal(Constant(new java.util.Date().toString))
}

14. reify
def assertImpl(c: Context)
(cond: c.Expr[Boolean], msg: c.Expr[String]) = {
import c.universe._
reify(if(!cond.splice){
throw new AssertionError(msg.splice)
})
}

15. Quasiquotes
def assertImpl(c: Context)
(cond: c.Expr[Boolean], msg: c.Expr[String]) = {
import c.universe._
q"if(!$cond){ throw new AssertionError($msg) }"
}

16. Future: scala.mata
● Metaprogramming toolkit for Scala
● Replace scala.reflect
○ More simple and safe
● Scala Macro will move to scala.meta based
○ Only Macro annotation is available by Macro
Paradise plug-in currently

17. Let's learn Macro!