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POStatic return type of Scala macros
 

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    <p>So I've got this macro:</p> <pre><code>import language.experimental.macros import scala.reflect.macros.Context class Foo class Bar extends Foo { def launchMissiles = "launching" } object FooExample { def foo: Foo = macro foo_impl def foo_impl(c: Context): c.Expr[Foo] = c.Expr[Foo](c.universe.reify(new Bar).tree) } </code></pre> <p>I've said three times that I want <code>foo</code> to return a <code>Foo</code>, and yet I can do the following (in 2.10.0-RC3):</p> <pre><code>scala&gt; FooExample.foo res0: Bar = Bar@4118f8dd scala&gt; res0.launchMissiles res1: String = launching </code></pre> <p>The same thing happens if I remove the type parameters on either <code>c.Expr</code>. If I really want to make sure that whoever's calling <code>foo</code> can't see that they're getting a <code>Bar</code>, I have to add a type ascription in the tree itself.</p> <p>This is actually pretty great—it means for example that I can point a macro at a schema of some sort and create an anonymous subclass of some <code>Vocabulary</code> class with member methods representing terms in the vocabulary, and these will be available on the returned object.</p> <p>I'd like to understand exactly what I'm doing, though, so I have a couple of questions. First, what is the return type on the <code>foo</code> method actually for? Is it just available for (optional) documentation? It clearly constrains the return type (e.g., I can't change it to <code>Int</code> in this case), and if I remove it entirely I get an error like this:</p> <pre><code>scala&gt; FooExample.foo &lt;console&gt;:8: error: type mismatch; found : Bar required: Nothing FooExample.foo ^ </code></pre> <p>But I <em>can</em> change it to <code>Any</code> and still get a statically typed <code>Bar</code> when I call <code>foo</code>.</p> <p>Second, is this behavior specified somewhere? This seems like a fairly elementary set of issues, but I haven't been able to search up a clear explanation or discussion.</p>
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    1. CO@som-snytt: But I still would have expected the return type on `foo` to have the final word (although I'm also glad it doesn't).
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    2. COThe return type annotation on `FooExample.foo` is very bizarre here. This is otherwise how I would expect macros to behave.
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    3. CO@drstevens: Agreed.
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