coffees .filter( c => c.id == 101 )
implemented using macros
handed over
to runtime
compile time (reification) runtime
SQL
SLICK
Scala SLICK Query
Queryable NoSQL
ASTs ASTs
frontend
Web
services
9](https://image.slidesharecdn.com/projectkeplercompiletimemetaprogrammingforscala-120420040241-phpapp01/85/Project-kepler-compile-time-metaprogramming-for-scala-9-320.jpg)
coffees .filter( c => c.id == 101 )
implemented as a macro that works on argument AST at compile time
def filter[T]( predicate: T => Boolean ) : Queryable[T] =
macro QueryableMacros.filter[T]
object QueryableMacros{
def filter[T:c.TypeTag] (c: scala.reflect.makro.Context)
(predicate : c.mirror.Expr[T => Boolean]) = …
}](https://image.slidesharecdn.com/projectkeplercompiletimemetaprogrammingforscala-120420040241-phpapp01/85/Project-kepler-compile-time-metaprogramming-for-scala-10-320.jpg)
![A macro body
object QueryableMacros{
// macro implementation signature
def filter[T:c.TypeTag] (c: scala.reflect.makro.Context)
(predicate: c.mirror.Expr[T => Boolean]) = {
// reify tree (ship to runtime)
val reifiedPredicate =
c.mirror.Expr[ reflect.mirror.Expr[T => Boolean] ](
c.reifyTree( c.reflectMirrorPrefix, projection.tree ) )
splicing
// splice into runtime call
c.reify{ translate("filter", c.prefix.eval, reifiedPredicate.eval ) }
}}
inlined at the call site (macro expansion),
translates queries further at runtime, e.g. to SQL](https://image.slidesharecdn.com/projectkeplercompiletimemetaprogrammingforscala-120420040241-phpapp01/85/Project-kepler-compile-time-metaprogramming-for-scala-11-320.jpg)
Project kepler compile time metaprogramming for scala
- 1. Scala Macros Eugene Burmako Jan Christopher Vogt London, 2012
- 2. Scala Macros Empower developers to extend the compiler and stay sane! This enables compile-time: – checks – processing – AST transformations – code generation – shipping of ASTs to runtime
- 3. Implementation Compile-time metaprogramming has long existed in Lisp, so it should be easy to implement, right?
- 4. Trickiness Homoiconicity for a language with syntax and types is hard. Being non-hygienic is evil, being hygienic imposes a complexity tax on the spec. Need quasiquotations to make AST manipulations bearable. Yet another concept.
- 5. Beauty Martin Odersky was suspicious: “And, we'd need to be convinced that it is beautifully simple, or it won't go into Scala”. Here is what convinced Martin:
- 6. Cats
- 7. The essence Scala reflection provides a slice of the compiler’s cake. The infrastructure is already there. Macros are (Tree*, Type*) => Tree functions. Just as simple as that. Hygiene itself is implemented by a macro. Hence we stay minimalistic and flexible.
- 9. SLICK overview val coffees = Queryable[Coffee]( /* db connection */ ) coffees .filter( c => c.id == 101 ) implemented using macros handed over to runtime compile time (reification) runtime SQL SLICK Scala SLICK Query Queryable NoSQL ASTs ASTs frontend Web services 9
- 10. Macros in SLICK val coffees = Queryable[Coffee]( /* db connection */ ) coffees .filter( c => c.id == 101 ) implemented as a macro that works on argument AST at compile time def filter[T]( predicate: T => Boolean ) : Queryable[T] = macro QueryableMacros.filter[T] object QueryableMacros{ def filter[T:c.TypeTag] (c: scala.reflect.makro.Context) (predicate : c.mirror.Expr[T => Boolean]) = … }
- 11. A macro body object QueryableMacros{ // macro implementation signature def filter[T:c.TypeTag] (c: scala.reflect.makro.Context) (predicate: c.mirror.Expr[T => Boolean]) = { // reify tree (ship to runtime) val reifiedPredicate = c.mirror.Expr[ reflect.mirror.Expr[T => Boolean] ]( c.reifyTree( c.reflectMirrorPrefix, projection.tree ) ) splicing // splice into runtime call c.reify{ translate("filter", c.prefix.eval, reifiedPredicate.eval ) } }} inlined at the call site (macro expansion), translates queries further at runtime, e.g. to SQL
- 12. Resulting macro expansion coffees .filter( c => c.id == 101 ) translate( "filter", coffees, Function( List("c"), Scala AST Apply( Select("c", "id"), "==", List( Literal(Constant(101))))) ) That are macros in SLICK
- 13. Applications in the compiler • Removed a compiler phase (LiftCode) • Made a 80% solution (manifests) a 99% solution (type tags) • Manifests à la carte – no longer hardcoded in Implicits.scala, it‘s just several macros (you can write your own implementations!) • Ideas: SourceLocations, SourceContexts, static requirements on the compiler and the compilation environment
- 14. Applications in the wild • SLICK • Macrocosm • Scalatex • Expecty • Scalaxy • Ideas: procedure typing with arrows, lens generation, ACP DSL, zero-overhead mocks
- 15. Future work • Debugging generated code • Untyped macros • Type macros • Macro annotations • Replacing the compiler with a macro
- 16. Future work • Debugging generated code • Untyped macros • Type macros • Macro annotations • Replacing the compiler with a macro Come on, just kidding about the last one!
- 17. Thank you! Questions and answers www.scalamacros.org