What's a macro?Learning by Examples

Takako Shimamoto BizReach, Inc

What to Talk About

• What are macros?

• Good thing about macros

• Actual use cases of macros

• Future of macros

Are you using macros?

If “Yes",

The Fab You!

Because

• Using macros is easy, developing macros is hard

What are macros?

• Code that generates code

• Not textual code generation

What are macros?

• An experimental feature of 2.10+

• Compiler invokes functions

Before macros

• Ad-hoc textual code generation by sbt plugin

• Compile-time AST operation by compiler plugin

Why macros are necessary?

• Code is made simple

• Efficiency improved

Macro is on your side!

To use macros?

• Need to be enabled explicitly

• import scala.language.experimental.macros

• -language:experimental.macros

Notes

• Macros are separate compilation

• Macro implementations need to be compiled before the main compilation

Def Macros

Def Macros

• Def macros replace well-typed terms with other well-typed terms

• Can contain arbitrary Scala constructs

Practical exampleScala Logging

Where?

logger.debug(s"Some $expensive message!")you call

if (logger.isDebugEnabled) logger.debug(s"Some $expensive message!")

replaced by macros

Macro definition• Starts with the conditional keyword macro

• Followed by a static macro implementation method

Implementation of the macro• Takes several parameter lists

Implementation of the macro• Takes several parameter lists

First comes a single parameter, of type

Context

Implementation of the macro• Takes several parameter lists

Next, followed by a list of parameters that have the same names as the macro

definition parameters

Macro definition

Implementation of the macro

• The original macro parameter has

• type T

• A macro implementation parameter has

• type c.Expr[T]

Quasiquotes• q"..." string interpolators that build code

• Unnecessary to directly implement the AST

• To use the quasiquotes from the macro, just write import c.universe._

Quasiquotes• Using the showRaw, it is possible to see the AST

Goodness

• The hard to comprehend notion of meta programming

• def macros are similar to the concept of a typed method call

Practical example

specs2

Where?

• To use macros to automatically generate matchersfor any complex type

• MatcherMacros trait

Where?

case class Cat(name: String, age: Int) !

// your test case val cat = Cat(name = "Tom", age = 2) cat must matchA[Cat].name("Tom")

you call

Where?

def matchACat( name: Matcher[String] = AlwaysMatcher(), age: Matcher[Int] = AlwaysMatcher() ): Matcher[Cat] = name ^^ {(cat: Cat) => cat.name} and age ^^ {(cat: Cat) => cat.age}

replaced by macros

Macro definition• Generics

• Has type parameters

Implementation of the macro• Type tags(actual type arguments) will be passed

along when the macro is expanded

Implementation of the macro• Type tags(actual type arguments) will be passed

along when the macro is expanded

Come with WeakTypeTag context bounds

Goodness

• DRY approach

• Usage is fairly simple

Blackbox vs Whitebox

• From 2.11

• Not implemented in 2.10 or in macro paradise

• in 2.12, only include blackbox macros

Blackbox vs Whitebox

• 2.10

• scala.reflect.macros.Context

• 2.11

• scala.reflect.macros.blackbox.Context

• scala.reflect.macros.whitebox.Context

Why distinction?• Type specification of macro is curious

class Foo class Bar extends Foo !

object FooExample { def foo: Foo = macro foo_impl def foo_impl(c: Context): c.Expr[Foo] = c.Expr[Foo](c.universe.reify(new Bar).tree) }

Scala 2.10

Why distinction?

• Type checking during macro expansion

• Not affect after expansion

scala> FooExample.foo res0: Bar = Bar@4118f8dd

Blackbox restrictions

• Type parameters of macro affect the type of after macro expansion

• When blackbox macro is used, techniques such as Implicit Macros will not work

No restrictions Whitebox

• Same as 2.10 def macros

• Everything that could be done with macros in 2.10 should be possible in 2.11

Practical example

Spire

Appendix - What is Spire?

• A numeric library for Scala

• Using features such as macros, type classes

• Fast and Precise

Where?

• Using string interpolation and macros, Spire provides convenient syntax for number types

• Evaluated at compile-time

Macro definition• As usual

string interpolation

Implementation of the macro• Syntax check at compile-time

Implementation of the macro• Syntax check at compile-time

Occur at compile-time if any errors

encounter

Goodness

• Static (compile-time) type check

• Runtime error decrease

But, has weak side.

Current state• Optimized towards compiler developers, not library

users

Complicated!!

Scala 2.12 (plan)

• Not introduce new features

• Bugfixes and stability improvements

• Mention later why reason…

scala.meta

• A new experimental API for metaprogramming

• Formerly known as Project Palladium

scala.meta

• Implemented in a library shipped separately from the official Scala distribution

• The first milestone release is scheduled for this fall

The goal of scala.meta

• Metaprogramming easy

• New API is going to greatly simplify writing macros

• Not require knowledge of compiler internals

Language model• All represented with trees

Types

NamesModifiers

…

Trees

scala.meta

Terms

Symbols

Language model

• Keeps all the information about the program

• Comments also remain

• No information is lost anymore

Manner of utilization

• Code generation

• Static type checking

• etc…

Summary

• Macros are actively used in the OSS library

• Can be more efficiently and safely programming

• Scala macros are evolving!

Thanks!!