Scalameta: Identify particular annotations - scala

Question

I want to auto-generate REST API models in Scala using scalameta annotation macros. Specifically, given:
#Resource case class User(
#get id : Int,
#get #post #patch name : String,
#get #post email : String,
registeredOn : Long
)
I want to generate:
object User {
case class Get(id: Int, name: String, email: String)
case class Post(name: String, email: String)
case class Patch(name: Option[String])
}
trait UserRepo {
def getAll: Seq[User.Get]
def get(id: Int): User.Get
def create(request: User.Post): User.Get
def replace(id: Int, request: User.Put): User.Get
def update(id: Int, request: User.Patch): User.Get
def delete(id: Int): User.Get
}
I have something working here: https://github.com/pathikrit/metarest
Specifically I am doing this:
import scala.collection.immutable.Seq
import scala.collection.mutable
import scala.annotation.{StaticAnnotation, compileTimeOnly}
import scala.meta._
class get extends StaticAnnotation
class put extends StaticAnnotation
class post extends StaticAnnotation
class patch extends StaticAnnotation
#compileTimeOnly("#metarest.Resource not expanded")
class Resource extends StaticAnnotation {
inline def apply(defn: Any): Any = meta {
val (cls: Defn.Class, companion: Defn.Object) = defn match {
case Term.Block(Seq(cls: Defn.Class, companion: Defn.Object)) => (cls, companion)
case cls: Defn.Class => (cls, q"object ${Term.Name(cls.name.value)} {}")
case _ => abort("#metarest.Resource must annotate a class")
}
val paramsWithAnnotation = for {
Term.Param(mods, name, decltype, default) <- cls.ctor.paramss.flatten
seenMods = mutable.Set.empty[String]
modifier <- mods if seenMods.add(modifier.toString)
(tpe, defArg) <- modifier match {
case mod"#get" | mod"#put" | mod"#post" => Some(decltype -> default)
case mod"#patch" =>
val optDeclType = decltype.collect({case tpe: Type => targ"Option[$tpe]"})
val defaultArg = default match {
case Some(term) => q"Some($term)"
case None => q"None"
}
Some(optDeclType -> Some(defaultArg))
case _ => None
}
} yield modifier -> Term.Param(Nil, name, tpe, defArg)
val models = paramsWithAnnotation
.groupBy(_._1.toString)
.map({case (verb, pairs) =>
val className = Type.Name(verb.stripPrefix("#").capitalize)
val classParams = pairs.map(_._2)
q"case class $className[..${cls.tparams}] (..$classParams)"
})
val newCompanion = companion.copy(
templ = companion.templ.copy(stats = Some(
companion.templ.stats.getOrElse(Nil) ++ models
))
)
Term.Block(Seq(cls, newCompanion))
}
}
I am unhappy with the following snip of code:
modifier match {
case mod"#get" | mod"#put" | mod"#post" => ...
case mod"#patch" => ...
case _ => None
}
The above code does "stringly" pattern matching on the annotations I have. Is there anyway to re-use the exact annotations I have to pattern match for these:
class get extends StaticAnnotation
class put extends StaticAnnotation
class post extends StaticAnnotation
class patch extends StaticAnnotation

Answer 1

It's possible to replace the mod#get stringly typed annotation with a get() extractor using a bit of runtime reflection (at compile time).
In addition, let's say we also want to allow users to fully qualify the annotation with #metarest.get or #_root_.metarest.get
All the following code examples assume import scala.meta._. The tree structure of #get, #metarest.get and #_root_.metarest.get are
# mod"#get".structure
res4: String = """ Mod.Annot(Ctor.Ref.Name("get"))
"""
# mod"#metarest.get".structure
res5: String = """
Mod.Annot(Ctor.Ref.Select(Term.Name("metarest"), Ctor.Ref.Name("get")))
"""
# mod"#_root_.metarest.get".structure
res6: String = """
Mod.Annot(Ctor.Ref.Select(Term.Select(Term.Name("_root_"), Term.Name("metarest")), Ctor.Ref.Name("get")))
"""
The selectors are either Ctor.Ref.Select or Term.Select and the names are either Term.Name or Ctor.Ref.Name.
Let's first create a custom selector extractor
object Select {
def unapply(tree: Tree): Option[(Term, Name)] = tree match {
case Term.Select(a, b) => Some(a -> b)
case Ctor.Ref.Select(a, b) => Some(a -> b)
case _ => None
}
}
Then create a few helper utilities
object ParamAnnotation {
/* isSuffix(c, a.b.c) // true
* isSuffix(b.c, a.b.c) // true
* isSuffix(a.b.c, a.b.c) // true
* isSuffix(_root_.a.b.c, a.b.c) // true
* isSuffix(d.c, a.b.c) // false
*/
def isSuffix(maybeSuffix: Term, fullName: Term): Boolean =
(maybeSuffix, fullName) match {
case (a: Name, b: Name) => a.value == b.value
case (Select(q"_root_", a), b: Name) => a.value == b.value
case (a: Name, Select(_, b)) => a.value == b.value
case (Select(aRest, a), Select(bRest, b)) =>
a.value == b.value && isSuffix(aRest, bRest)
case _ => false
}
// Returns true if `mod` matches the tree structure of `#T`
def modMatchesType[T: ClassTag](mod: Mod): Boolean = mod match {
case Mod.Annot(term: Term.Ref) =>
isSuffix(term, termRefForType[T])
case _ => false
}
// Parses `T.getClass.getName` into a Term.Ref
// Uses runtime reflection, but this happens only at compile time.
def termRefForType[T](implicit ev: ClassTag[T]): Term.Ref =
ev.runtimeClass.getName.parse[Term].get.asInstanceOf[Term.Ref]
}
With this setup, we can add a companion object to the get definition with an
unapply boolean extractor
class get extends StaticAnnotation
object get {
def unapply(mod: Mod): Boolean = ParamAnnotation.modMatchesType[get](mod)
}
Doing the same for post and put, we can now write
// before
case mod"#get" | mod"#put" | mod"#post" => Some(decltype -> default)
// after
case get() | put() | post() => Some(decltype -> default)
Note that this approach will still not work if the user renames for example get on import
import metarest.{get => GET}
I would recommend aborting if an annotation does not match what you expected
// before
case _ => None
// after
case unexpected => abort("Unexpected modifier $unexpected. Expected one of: put, get post")
PS. The object get { def unapply(mod: Mod): Boolean = ... } part is boilerplate that could be generated by some #ParamAnnotation macro annotation, for example #ParamAnnotion class get extends StaticAnnotation