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POGetting a structural type with an anonymous class's methods from a macro
 

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  1. POGetting a structural type with an anonymous class's methods from a macro
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    <p>Suppose we want to write a macro that defines an anonymous class with some type members or methods, and then creates an instance of that class that's statically typed as a structural type with those methods, etc. This is possible with the macro system in 2.10.0, and the type member part is extremely easy:</p> <pre><code>object MacroExample extends ReflectionUtils { import scala.language.experimental.macros import scala.reflect.macros.Context def foo(name: String): Any = macro foo_impl def foo_impl(c: Context)(name: c.Expr[String]) = { import c.universe._ val Literal(Constant(lit: String)) = name.tree val anon = newTypeName(c.fresh) c.Expr(Block( ClassDef( Modifiers(Flag.FINAL), anon, Nil, Template( Nil, emptyValDef, List( constructor(c.universe), TypeDef(Modifiers(), newTypeName(lit), Nil, TypeTree(typeOf[Int])) ) ) ), Apply(Select(New(Ident(anon)), nme.CONSTRUCTOR), Nil) )) } } </code></pre> <p>(Where <code>ReflectionUtils</code> is a <a href="https://gist.github.com/4552471" rel="nofollow noreferrer">convenience trait</a> that provides my <code>constructor</code> method.)</p> <p>This macro lets us specify the name of the anonymous class's type member as a string literal:</p> <pre><code>scala&gt; MacroExample.foo("T") res0: AnyRef{type T = Int} = $1$$1@7da533f6 </code></pre> <p>Note that it's appropriately typed. We can confirm that everything's working as expected:</p> <pre><code>scala&gt; implicitly[res0.T =:= Int] res1: =:=[res0.T,Int] = &lt;function1&gt; </code></pre> <p>Now suppose that we try to do the same thing with a method:</p> <pre><code>def bar(name: String): Any = macro bar_impl def bar_impl(c: Context)(name: c.Expr[String]) = { import c.universe._ val Literal(Constant(lit: String)) = name.tree val anon = newTypeName(c.fresh) c.Expr(Block( ClassDef( Modifiers(Flag.FINAL), anon, Nil, Template( Nil, emptyValDef, List( constructor(c.universe), DefDef( Modifiers(), newTermName(lit), Nil, Nil, TypeTree(), c.literal(42).tree ) ) ) ), Apply(Select(New(Ident(anon)), nme.CONSTRUCTOR), Nil) )) } </code></pre> <p>But when we try it out, we don't get a structural type:</p> <pre><code>scala&gt; MacroExample.bar("test") res1: AnyRef = $1$$1@da12492 </code></pre> <p>But if we stick an extra anonymous class in there:</p> <pre><code>def baz(name: String): Any = macro baz_impl def baz_impl(c: Context)(name: c.Expr[String]) = { import c.universe._ val Literal(Constant(lit: String)) = name.tree val anon = newTypeName(c.fresh) val wrapper = newTypeName(c.fresh) c.Expr(Block( ClassDef( Modifiers(), anon, Nil, Template( Nil, emptyValDef, List( constructor(c.universe), DefDef( Modifiers(), newTermName(lit), Nil, Nil, TypeTree(), c.literal(42).tree ) ) ) ), ClassDef( Modifiers(Flag.FINAL), wrapper, Nil, Template(Ident(anon) :: Nil, emptyValDef, constructor(c.universe) :: Nil) ), Apply(Select(New(Ident(wrapper)), nme.CONSTRUCTOR), Nil) )) } </code></pre> <p>It works:</p> <pre><code>scala&gt; MacroExample.baz("test") res0: AnyRef{def test: Int} = $2$$1@6663f834 scala&gt; res0.test res1: Int = 42 </code></pre> <p>This is extremely handy—it lets you do things like <a href="https://github.com/travisbrown/rillit" rel="nofollow noreferrer">this</a>, for example—but I don't understand why it works, and the type member version works, but not <code>bar</code>. I know this <a href="https://stackoverflow.com/q/13669974/334519">may not be defined behavior</a>, but does it make any sense? Is there an cleaner way to get a structural type (with the methods on it) from a macro?</p>
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    1. COInterestingly enough, if you write the same code in REPL instead of generating it in a macro, it works: scala> { final class anon { def x = 2 }; new anon } res1: AnyRef{def x: Int} = anon$1@5295c398. Thanks for the report! I'll take a look this week.
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    2. CONote that I've filed an issue [here](https://issues.scala-lang.org/browse/SI-6992).
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      1. POGetting a structural type with an anonymous class's methods from a macro
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    3. CONope, not a blocker, thanks—the extra anonymous class trick has worked for me whenever I needed it. I just noticed a couple of upvotes on the question and was curious about the status.
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