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POScala Macros: Making a Map out of fields of a class in Scala
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copied!<p>Let's say that I have a lot of similar data classes. Here's an example class <code>User</code> which is defined as follows:</p> <pre><code>case class User (name: String, age: Int, posts: List[String]) { val numPosts: Int = posts.length ... def foo = "bar" ... } </code></pre> <p>I am interested in automatically creating a method (<strong>at compile time</strong>) that returns a <code>Map</code> in a way that each field name is mapped to its value when it is called in runtime. For the example above, let's say that my method is called <code>toMap</code>:</p> <pre><code>val myUser = User("Foo", 25, List("Lorem", "Ipsum")) myUser.toMap </code></pre> <p>should return</p> <pre><code>Map("name" -> "Foo", "age" -> 25, "posts" -> List("Lorem", "Ipsum"), "numPosts" -> 2) </code></pre> <p>How would you do this with macros? </p> <p>Here's what I have done: First, I created a <code>Model</code> class as a superclass for all of my data classes and implemented the method in there like this:</p> <pre><code>abstract class Model { def toMap[T]: Map[String, Any] = macro toMap_impl[T] } class User(...) extends Model { ... } </code></pre> <p>Then I defined a macro implementation in a separate <code>Macros</code> object:</p> <pre><code>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)) } } </code></pre> <p>Yet I get this error from sbt when I try to compile it:</p> <pre><code>[error] /Users/emre/workspace/DynamoReflection/core/src/main/scala/dynamo/Main.scala:9: not found: value posts [error] foo.getMap[User] [error] ^ </code></pre> <p>Macros.scala is being compiled first. Here is the snippet from my Build.scala:</p> <pre><code>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) </code></pre> <p>What am I doing wrong? I think that the compiler tries to evaluate the field identifiers too when it creates the expression, but I don't know how to return them properly in the expression. Could you show me how to do that?</p> <p>Thanks very much in advance.</p>

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