An exploration of Scala macros.
This branch has been adapted for the new syntax proposed in SIP-16
Prerequisites
SBT 0.12.0
Use the latest launcher. You might want to start SBT without using your global ~/.sbt folder with incompatible plugins.
The easiest way to obtain the right SBT version, and to use a separate .sbt directory is to use Paul Phillips' sbt-extras launcher script.
Now, the macros!
Compiler Diagnostics / Tree Manipulation
> console
[info] Compiling 1 Scala source to /Users/jason/code/scala-spock/target/scala-2.10.0-SNAPSHOT/classes...
[info] Starting scala interpreter...
[info]
Welcome to Scala version 2.10.0-M1-0362-g2fe570291a-2012-02-18 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_29).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import com.github.retronym.macrocosm.Macrocosm._
import com.github.retronym.macrocosm.Macrocosm._
scala> desugar("foo".map(_ + 1))
res0: String = scala.this.Predef.augmentString("foo").map[Int, Any](((x$1: Char) => x$1.+(1)))(scala.this.Predef.fallbackStringCanBuildFrom[Int])
The following work without macrocosm.
scala> val t = reflect.mirror.reify("abc".reverse)
t: reflect.mirror.Expr[String] = Expr[null](scala.this.Predef.augmentString("abc").reverse)
scala> reflect.mirror.showRaw(t.tree)
res3: String = Select(Apply(Select(Select(This(newTypeName("scala")), newTermName("Predef")), newTermName("augmentString")), List(Literal(Constant("abc")))), newTermName("reverse"))
scala> val tb = new reflect.runtime.Mirror.ToolBox()
tb: reflect.runtime.Mirror.ToolBox = scala.reflect.runtime.ToolBoxes$ToolBox@55b6ed96
scala> tb.typeCheck(t.tree)
res5: reflect.mirror.Tree = scala.this.Predef.augmentString("abc").reverse
scala> .tpe
res6: reflect.mirror.Type = String
scala> tb.runExpr(t.tree)
res8: Any = cba
Especially fun is using desugar to see what the other macros here have done!
scala> desugar{def foo[T: Numeric](t: T) = t - t}
res7: String =
{
def foo[T >: Nothing <: Any](t: T)(implicit evidence$1: Numeric[T]): T = evidence$1.minus(t, t);
()
}
scala> desugar{var i = 0; cfor(0)(_ < 10, _ + 1)((a: Int) => i += 1)}
res8: String =
{
var i: Int = 0;
{
var $a: Int = 0;
while$1(){
if ({
val x$1: Int = $a;
x$1.<(10)
})
{
{
{
val a: Int = $a;
i = i.+(1)
};
$a = {
val x$2: Int = $a;
x$2.+(1)
}
};
while$1()
}
else
()
}
}
}
Logging / Assertions
scala> log("".isEmpty)
"".isEmpty() = true
res1: Boolean = true
scala> assert1("foo".reverse == "off")
java.lang.AssertionError: assertion failed: scala.this.Predef.augmentString("foo").reverse.==("off")
at scala.Predef$.assert(Predef.scala:161)
at .<init>(<console>:10)
scala> def plus(a: Int, b: Int) = a + b
plus: (a: Int, b: Int)Int
scala> trace(plus(1, plus(2, plus(3, 4))))
$line1.$read.$iw.$iw.plus(3, 4) = 7
$line1.$read.$iw.$iw.plus(2, $line1.$read.$iw.$iw.plus(3, 4)) = 9
$line1.$read.$iw.$iw.plus(1, $line1.$read.$iw.$iw.plus(2, $line1.$read.$iw.$iw.plus(3, 4))) = 10
res3: Int = 10
New literals
scala> b"101010"
res4: Int = 42
scala> b"102"
<console>:11: error: exception during macro expansion: invalid binary literal
b"102"
^
Statically Checked Regex
scala> regex(".*")
res0: scala.util.matching.Regex = .*
scala> regex("{")
<console>:11: error: exception during macro expansion: Illegal repetition
{
regex("{")
^
High Performance Loops
Translated to while loops, eliminate the indirection through FunctionN, and avoids boxing.
scala> cfor(0)(_ < 10, _ + 2)(println(_))
0
2
4
6
8
scala> val as = Array(1, 2, 3)
as: Array[Int] = Array(1, 2, 3)
scala> iteratorForeach(Iterator(1, 2, 3, 4, 5))(println(_))
1
2
3
4
5
Implicit Wrapper Elimination
Rewrite enrich[T](lhs)(numericInstance).*(rhs) to numericInstance.plus(lhs, rhs)
by defining * as a macro def.
scala> import com.github.retronym.macrocosm.Macrocosm._
import com.github.retronym.macrocosm.Macrocosm._
scala> object A { def foo[T: Numeric](t: T) = (-t * t).abs }
public java.lang.Object foo(java.lang.Object, scala.math.Numeric);
Code:
Stack=4, Locals=3, Args_size=3
0: aload_2
1: aload_2
2: aload_2
3: aload_1
4: invokeinterface #21, 2; //InterfaceMethod scala/math/Numeric.negate:(Ljava/lang/Object;)Ljava/lang/Object;
9: aload_1
10: invokeinterface #25, 3; //InterfaceMethod scala/math/Numeric.times:(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
15: invokeinterface #28, 2; //InterfaceMethod scala/math/Numeric.abs:(Ljava/lang/Object;)Ljava/lang/Object;
20: areturn
LineNumberTable:
line 10: 2
Dynamic Lens Creation
This even works for case classes defined in the current compile run.
scala> case class Person(name: String, age: Int)
defined class Person
scala> val nameLens = lens[Person].name
dynatype: com.github.retronym.macrocosm.Macrocosm.lens[Person].applyDynamic("name")
TypeApply(Select(Select(Select(Select(Select(Ident(newTermName("com")), newTermName("github")), newTermName("retronym")), newTermName("macrocosm")), newTermName("Macrocosm")), newTermName("lens")), List(TypeTree().setType(Person)))
nameLens: (Person => String, (Person, String) => Person) = (<function1>,<function2>)
scala> val p = Person("brett", 21)
p: Person = Person(brett,21)
scala> val nameLens = lens[Person].name
dynatype: com.github.retronym.macrocosm.Macrocosm.lens[Person].applyDynamic("name")()
TypeApply(Select(Select(Select(Select(Select(Ident(newTermName("com")), newTermName("github")), newTermName("retronym")), newTermName("macrocosm")), newTermName("Macrocosm")), newTermName("lens")), List(TypeTree().setType(Person)))
nameLens: (Person => String, (Person, String) => Person) = (<function1>,<function2>)
scala> nameLens._1(p)
res1: String = brett
scala> nameLens._2(p, "bill")
res2: Person = Person(bill,21)
scala> lens[Person].namexx(())
dynatype: com.github.retronym.macrocosm.Macrocosm.lens[Person].applyDynamic("namexx")()
TypeApply(Select(Select(Select(Select(Select(Ident(newTermName("com")), newTermName("github")), newTermName("retronym")), newTermName("macrocosm")), newTermName("Macrocosm")), newTermName("lens")), List(TypeTree().setType(Person)))
error: exception during macro expansion: value namexx is not a member of Person