scalameta-tut-macro1.scala

// @param backend - parameter for `cache` annotation
class cache[K, V](backend: SyncCache[K, V]) extends scala.annotation.StaticAnnotation {
  
  // @param defn - the annotated method (it can also be other scala building block like class, but we are restricting here    
  //               using some checks below
  inline def apply(defn: Any): Any = meta {

    defn match {
      // this annotation should only be annotate on `method`, represented as `Defn.Def` in scalameta's AST
      case defn: Defn.Def =>
        
        // this represent the instatiated annotation, ie. `@cache(xxx)`
        this match {
        
          // Quasiquote in action, it's a way to pattern match scala AST
          //  - `new`               match instantiation
          //  - `$_`                match the name of the annotation
          //  - `[..$tpr]`          match type params and bind to `tpr`
          //  - `($backendParam)`   match SINGLE argument used in instatiation and bind to `backendParam`
          //                        Note: To match multiple arguments or multiple arguments list (curried), you need different                                           syntax
          case q"new $_[..$tpr]($backendParam)" =>
          
            // we use the element we captured to generate code we want, we will look into details next
            val body: Term = CacheMacroImpl.expand(tpr, backendParam, defn)
            
            // we only want to replace the method body, so this will do
            defn.copy(body = body)
          case x =>
            abort(s"Unrecognized pattern $x")
        }

      case _ =>
        abort("This annotation only works on `def`")    // abort if annotated to anything other than methods
    }
  }
}