Scala宏的静态返回类型 [英] Static return type of Scala macros

问题描述

所以我有这个宏:

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)
}

我已经说过三遍了，我希望foo返回Foo，但是我可以执行以下操作(在2.10.0-RC3中):

I've said three times that I want foo to return a Foo, and yet I can do the following (in 2.10.0-RC3):

scala> FooExample.foo
res0: Bar = Bar@4118f8dd

scala> res0.launchMissiles
res1: String = launching

如果我删除任一c.Expr上的类型参数，也会发生同样的事情.如果我真的想确保呼叫foo的人看不到他们正在获得Bar，则必须在树本身中添加类型说明.

The same thing happens if I remove the type parameters on either c.Expr. If I really want to make sure that whoever's calling foo can't see that they're getting a Bar, I have to add a type ascription in the tree itself.

这实际上是非常棒的-例如，这意味着我可以将宏指向某种模式，并创建某个Vocabulary类的匿名子类，并且该类的成员方法表示词汇中的术语，这些将可用在返回的对象上.

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 Vocabulary class with member methods representing terms in the vocabulary, and these will be available on the returned object.

不过，我想确切地了解自己在做什么，所以我有几个问题.首先，foo方法的实际返回类型是什么?它仅可用于(可选)文档吗?它显然限制了返回类型(例如，在这种情况下，我无法将其更改为Int)，如果我将其完全删除，则会收到如下错误:

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 foo method actually for? Is it just available for (optional) documentation? It clearly constrains the return type (e.g., I can't change it to Int in this case), and if I remove it entirely I get an error like this:

scala> FooExample.foo
<console>:8: error: type mismatch;
 found   : Bar
 required: Nothing
              FooExample.foo
                         ^

但是我可以将其更改为Any，当我调用foo时仍然得到静态类型的Bar.

But I can change it to Any and still get a statically typed Bar when I call foo.

第二，此行为是否在某处指定?这似乎是一组相当基本的问题，但我无法找到清晰的解释或讨论.

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.

推荐答案

此行为虽未指定，但是有意的，尽管可能会引起混淆.我们计划详细说明返回类型在宏签名中的作用，但目前我觉得灵活性是一件好事.

This behavior is underspecified but intended, though it might appear confusing. We plan to elaborate on the role of return type in macro signatures, but at the moment I feel like the flexibility is a good thing to have.

有时行为也不一致，例如当宏在类型推断的中间被捕获时，将使用其静态签名(即示例中的Foo)，而不是实际扩展的类型.这是因为宏扩展被故意延迟，直到完成类型推断为止(这样宏实现才能看到推断的类型，而不是类型var).这是一个折衷方案，不一定是最好的方案，因此我们计划尽快进行重新评估:

Also at times the behavior is inconsistent, e.g. when the macro is caught in the middle of type inference, its static signature will be used (i.e. Foo in your example), not the type of the actual expansion. That's because macro expansion is intentionally delayed until type inference is done (so that macro implementations get to see inferred types, not type vars). This is a trade-off and not necessarily the best one, so we're planning to revisit it soon: https://issues.scala-lang.org/browse/SI-6755.

该部门的另一个问题是隐式宏.当隐式宏的返回类型是通用的并且需要从所请求的隐式值的类型中推断出时，就会发生不好的事情.这使得当前无法使用宏生成类型标签: https://issues.scala- lang.org/browse/SI-5923 .

Another problem in this department is with implicit macros. When the return type of an implicit macro is generic and needs to be inferred from the requested type of an implicit value, bad things happen. This makes it currently impossible to use macros to generate type tags: https://issues.scala-lang.org/browse/SI-5923.

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