假设我有很多相似的数据类。下面是一个叫做User
的示例类定义:
case class User (name: String, age: Int, posts: List[String]) {
val numPosts: Int = posts.length
...
def foo = "bar"
...
}
我对在编译时自动创建一个方法很感兴趣,该方法返回一个Map
,使得每个字段名在运行时被调用时都能映射到其值。以上面的示例为例,假设我的方法名为toMap
:
val myUser = User("Foo", 25, List("Lorem", "Ipsum"))
myUser.toMap
应该返回
Map("name" -> "Foo", "age" -> 25, "posts" -> List("Lorem", "Ipsum"), "numPosts" -> 2)
你如何使用宏来完成这个任务?
以下是我所做的步骤:首先,我创建了一个 Model
类作为所有数据类的超类,并在其中实现了该方法:
abstract class Model {
def toMap[T]: Map[String, Any] = macro toMap_impl[T]
}
class User(...) extends Model {
...
}
然后我在单独的Macros
对象中定义了一个宏实现:
object Macros {
import scala.language.experimental.macros
import scala.reflect.macros.Context
def getMap_impl[T: c.WeakTypeTag](c: Context): c.Expr[Map[String, Any]] = {
import c.universe._
val tpe = weakTypeOf[T]
// Filter members that start with "value", which are val fields
val members = tpe.members.toList.filter(m => !m.isMethod && m.toString.startsWith("value"))
// Create ("fieldName", field) tuples to construct a map from field names to fields themselves
val tuples =
for {
m <- members
val fieldString = Literal(Constant(m.toString.replace("value ", "")))
val field = Ident(m)
} yield (fieldString, field)
val mappings = tuples.toMap
/* Parse the string version of the map [i.e. Map("posts" -> (posts), "age" -> (age), "name" -> (name))] to get the AST
* for the map, which is generated as:
*
* Apply(Ident(newTermName("Map")),
* List(
* Apply(Select(Literal(Constant("posts")), newTermName("$minus$greater")), List(Ident(newTermName("posts")))),
* Apply(Select(Literal(Constant("age")), newTermName("$minus$greater")), List(Ident(newTermName("age")))),
* Apply(Select(Literal(Constant("name")), newTermName("$minus$greater")), List(Ident(newTermName("name"))))
* )
* )
*
* which is equivalent to Map("posts".$minus$greater(posts), "age".$minus$greater(age), "name".$minus$greater(name))
*/
c.Expr[Map[String, Any]](c.parse(mappings.toString))
}
}
当我尝试编译时,sbt会出现以下错误:
[error] /Users/emre/workspace/DynamoReflection/core/src/main/scala/dynamo/Main.scala:9: not found: value posts
[error] foo.getMap[User]
[error] ^
首先编译的是 Macros.scala。 这是我的 Build.scala 中的代码片段:
lazy val root: Project = Project(
"root",
file("core"),
settings = buildSettings
) aggregate(macros, core)
lazy val macros: Project = Project(
"macros",
file("macros"),
settings = buildSettings ++ Seq(
libraryDependencies <+= (scalaVersion)("org.scala-lang" % "scala-reflect" % _))
)
lazy val core: Project = Project(
"core",
file("core"),
settings = buildSettings
) dependsOn(macros)
我做错了什么?我认为编译器在创建表达式时试图评估字段标识符,但我不知道如何正确地在表达式中返回它们。你能向我展示如何做吗? 非常感谢。
Select(c.prefix.tree, newTermName("posts"))
,而不是仅仅使用Ident(newTermName(posts))
。 - Eugene Burmakothis
),而是内联到调用站点中。 - Eugene Burmako