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Scala def macros and Java interoperability

Most of the time Scala-Java interoperability works pretty well from a Scala application developer perspective: you can use a wealth of Java class libraries in your Scala programs with fairly little effort. Simply using JavaConversions and maybe a few custom wrappers usually gets you pretty far. Sure, there can be some friction resulting from the use of different programming paradigms and mutable data structures, but if you’re a pragmatist re-using Java code in Scala is nevertheless quite feasible. Scala version 2.10 saw the introduction of an experimental language feature called macros. Specification work on Scala macros recognizes different flavors of macros, but versions 2.10 and 2.11, as well as the future version 2.12, support only def macros. This macro variety behaves similar to methods except that def macro invocations are expanded at compile time. Here’s how EPFL Scala team member and “Scala macros guy” Eugene Burmako characterizes def macros:

If, during type-checking, the compiler encounters an application of the macro m(args), it will expand that application by invoking the corresponding macro implementation method, with the abstract-syntax trees of the argument expressions args as arguments. The result of the macro implementation is another abstract syntax tree, which will be inlined at the call site and will be type-checked in turn.

Def macros resemble C/C++ pre-preprocessor macros, the m4 macro processor and similar in that the result of macro application will be inlined at the call site. A notable difference is that Scala macros are well integrated into the language meaning e.g. that the results of macro expansion are type-checked. But let’s look at this from the code perspective. I’ve implemented a “hello, world” macro and an object called MyReusableService that defines two methods: a regular method and one implemented as a macro. Two objects, ScalaClient and JavaClient, invoke methods on MyReusableService. Here’s what happens when compiling Java code that tries to invoke a macro method on a Scala object:

~ ᐅ sbt 'runMain com.practicingtechie.gist.macros.JavaClient'
...
[error] /Users/marko/blog-gists/macros-interop/src/main/java/com/practicingtechie/gist/macros/JavaClient.java:6: cannot find symbol
[error] symbol: method macroMethod()
[error] location: class com.practicingtechie.gist.macros.MyReusableService
[error] MyReusableService.macroMethod

Method macroMethod is defined by MyReusableService Scala object, but the method is not visible when inspecting the disassembled class file:

~ ᐅ javap -cp target/scala-2.11/classes com.practicingtechie.gist.macros.MyReusableService$
Compiled from "MyReusableService.scala"
public final class com.practicingtechie.gist.macros.MyReusableService$ {
  public static final com.practicingtechie.gist.macros.MyReusableService$ MODULE$;
  public static {};
  public void regularMethod();
}

After removing the macroMethod invocation JavaClient is able to compile. ScalaClient, however, is more interesting. Here’s macro debugging output from running the code (with “-Ymacro-debug-lite” argument):

~ ᐅ sbt 'runMain com.practicingtechie.gist.macros.ScalaClient'
...
[info] Compiling 2 Scala sources and 1 Java source to /Users/marko/blog-gists/macros-interop/target/scala-2.11/classes...
performing macro expansion MyReusableService.macroMethod at source-/Users/marko/blog-gists/macros-interop/src/main/scala/com/practicingtechie/gist/macros/ScalaClient.scala,line-7,offset=158
println("Hello macro world")
Apply(Ident(TermName("println")), List(Literal(Constant("Hello macro world"))))
[info] Running com.practicingtechie.gist.macros.ScalaClient
Hello, from regular method
Hello macro world

In the above extract we can see the location where the macro was applied, as well as results of macro expansion, both as Scala code and as well as abstract-syntax tree (AST) representation.

Finally, we can see macro expansion results expanded and compiled into bytecode at the ScalaClient call site:

~ ᐅ javap -c -cp target/scala-2.11/classes com.practicingtechie.gist.macros.ScalaClient$
...
  public void main(java.lang.String[]);
    Code:
       0: getstatic     #19                 // Field com/practicingtechie/gist/macros/MyReusableService$.MODULE$:Lcom/practicingtechie/gist/macros/MyReusableService$;
       3: invokevirtual #22                 // Method com/practicingtechie/gist/macros/MyReusableService$.regularMethod:()V
       6: getstatic     #27                 // Field scala/Predef$.MODULE$:Lscala/Predef$;
       9: ldc           #29                 // String Hello macro world
      11: invokevirtual #33                 // Method scala/Predef$.println:(Ljava/lang/Object;)V
...

Scala def macros are a very interesting language feature that’s planned to be officially supported in a future Scala version. Def macros are implemented by the Scala compiler, so a function or method whose definition is macro based won’t be accessible in Java code. This is because unlike ordinary function or method invocations the result of the macro application gets expanded at the call site. Still, Scala functions or methods that simply invoke macros from within their body can nonetheless be called from Java code. Depending on how def macros are used, they can sometimes hinder reuse of Scala code from Java or other JVM-based languages.

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