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POHowto model named parameters in method invocations with Scala macros?
 

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  1. POHowto model named parameters in method invocations with Scala macros?
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    <p>There are use cases where it is useful to create a copy of an object which is an instance of a case class of a set of case classes, which have a specific value in common.</p> <p>For example let's consider the following case classes:</p> <pre><code>case class Foo(id: Option[Int]) case class Bar(arg0: String, id: Option[Int]) case class Baz(arg0: Int, id: Option[Int], arg2: String) </code></pre> <p>Then <code>copy</code> can be called on each of these case class instances:</p> <pre><code>val newId = Some(1) Foo(None).copy(id = newId) Bar("bar", None).copy(id = newId) Baz(42, None, "baz").copy(id = newId) </code></pre> <p>As described <a href="http://www.scala-lang.org/node/4620" rel="nofollow noreferrer">here</a> and <a href="https://stackoverflow.com/questions/12370244/case-class-copy-method-with-superclass">here</a> there is no simple way to abstract this like this:</p> <pre><code>type Copyable[T] = { def copy(id: Option[Int]): T } // THIS DOES *NOT* WORK FOR CASE CLASSES def withId[T &lt;: Copyable[T]](obj: T, newId: Option[Int]): T = obj.copy(id = newId) </code></pre> <p>So I created a scala macro, which does this job (almost):</p> <pre><code>import scala.reflect.macros.Context object Entity { import scala.language.experimental.macros import scala.reflect.macros.Context def withId[T](entity: T, id: Option[Int]): T = macro withIdImpl[T] def withIdImpl[T: c.WeakTypeTag](c: Context)(entity: c.Expr[T], id: c.Expr[Option[Int]]): c.Expr[T] = { import c.universe._ val currentType = entity.actualType // reflection helpers def equals(that: Name, name: String) = that.encoded == name || that.decoded == name def hasName(name: String)(implicit method: MethodSymbol) = equals(method.name, name) def hasReturnType(`type`: Type)(implicit method: MethodSymbol) = method.typeSignature match { case MethodType(_, returnType) =&gt; `type` == returnType } def hasParameter(name: String, `type`: Type)(implicit method: MethodSymbol) = method.typeSignature match { case MethodType(params, _) =&gt; params.exists { param =&gt; equals(param.name, name) &amp;&amp; param.typeSignature == `type` } } // finding method entity.copy(id: Option[Int]) currentType.members.find { symbol =&gt; symbol.isMethod &amp;&amp; { implicit val method = symbol.asMethod hasName("copy") &amp;&amp; hasReturnType(currentType) &amp;&amp; hasParameter("id", typeOf[Option[Int]]) } } match { case Some(symbol) =&gt; { val method = symbol.asMethod val param = reify(( c.Expr[String](Literal(Constant("id"))).splice, id.splice)).tree c.Expr( Apply( Select( reify(entity.splice).tree, newTermName("copy")), List( /*id.tree*/ ))) } case None =&gt; c.abort(c.enclosingPosition, currentType + " needs method 'copy(..., id: Option[Int], ...): " + currentType + "'") } } } </code></pre> <p>The last argument of <code>Apply</code> (see bottom of above code block) is a List of parameters (here: parameters of method 'copy'). How can the given <code>id</code> of type <code>c.Expr[Option[Int]]</code> be passed as named parameter to the copy method with the help of the new macro API?</p> <p>In particular the following macro expression</p> <pre><code>c.Expr( Apply( Select( reify(entity.splice).tree, newTermName("copy")), List(/*?id?*/))) </code></pre> <p>should result in</p> <pre><code>entity.copy(id = id) </code></pre> <p>so that the following holds</p> <pre><code>case class Test(s: String, id: Option[Int] = None) // has to be compiled by its own object Test extends App { assert( Entity.withId(Test("scala rulz"), Some(1)) == Test("scala rulz", Some(1))) } </code></pre> <p>The missing part is denoted by the placeholder <code>/*?id?*/</code>.</p>
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