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    <p>Here's an implementation that's also a little more generic:</p> <pre><code>import scala.language.experimental.macros object WithIdExample { import scala.reflect.macros.Context def withId[T, I](entity: T, id: I): T = macro withIdImpl[T, I] def withIdImpl[T: c.WeakTypeTag, I: c.WeakTypeTag](c: Context)( entity: c.Expr[T], id: c.Expr[I] ): c.Expr[T] = { import c.universe._ val tree = reify(entity.splice).tree val copy = entity.actualType.member(newTermName("copy")) val params = copy match { case s: MethodSymbol if (s.paramss.nonEmpty) =&gt; s.paramss.head case _ =&gt; c.abort(c.enclosingPosition, "No eligible copy method!") } c.Expr[T](Apply( Select(tree, copy), params.map { case p if p.name.decoded == "id" =&gt; reify(id.splice).tree case p =&gt; Select(tree, p.name) } )) } } </code></pre> <p>It'll work on any case class with a member named <code>id</code>, no matter what its type is:</p> <pre><code>scala&gt; case class Bar(arg0: String, id: Option[Int]) defined class Bar scala&gt; case class Foo(x: Double, y: String, id: Int) defined class Foo scala&gt; WithIdExample.withId(Bar("bar", None), Some(2)) res0: Bar = Bar(bar,Some(2)) scala&gt; WithIdExample.withId(Foo(0.0, "foo", 1), 2) res1: Foo = Foo(0.0,foo,2) </code></pre> <p>If the case class doesn't have an <code>id</code> member, <code>withId</code> will compile—it just won't do anything. If you want a compile error in that case, you can add an extra condition to the match on <code>copy</code>.</p> <hr> <p>Edit: As Eugene Burmako just pointed out <a href="https://twitter.com/xeno_by/status/270495257152593920" rel="noreferrer">on Twitter</a>, you can write this a little more naturally using <code>AssignOrNamedArg</code> at the end:</p> <pre><code>c.Expr[T](Apply( Select(tree, copy), AssignOrNamedArg(Ident("id"), reify(id.splice).tree) :: Nil )) </code></pre> <p>This version won't compile if the case class doesn't have an <code>id</code> member, but that's more likely to be the desired behavior anyway.</p>
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    1. POHowto model named parameters in method invocations with Scala macros?
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    1. USTravis Brown
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    1. POHowto model named parameters in method invocations with Scala macros?
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    1. COThank you, I like the conciseness of this solution. It applies well to my use case. Perhaps the s.paramss.head part needs an extra check for nullary methods (= methods with no argument list), i.e. when s.paramss returns List() / Nil. But the result is the same: the macro cannot be applied.
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      1. USDaniel Dietrich
    2. CO@DanielDietrich: Good point, and I've added that check, but note that this is just a sketch and there's still at least one similar assumption in the revised version (that there's only one method named `copy`). Fortunately the worst that could happen is a somewhat confusing compile-time error.
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    3. COYes, you're right. As you said in your first post, a check for the existence of param id could be made. In the current solution there is already a compile error, if the param id is missing. To get a more verbose compiler error message, I would change the if-guard of the pattern match to (s.paramss.flatten.map(_.name).contains(newTermName("id"))). With this, nullary methods are also catched.
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