Scala理解中val的作用域规则是什么 [英] What are the scoping rules for vals in Scala for-comprehensions
问题描述
当我对理解使用val时，得到警告:
警告:不建议使用val关键字进行理解
尽管在规范的语法附录中进行了说明.
这表明当我做类似的事情
for (x <- xs; a = x)
我并没有真正引入变量，例如，如果我做类似的事情
for (x <- xs) yield { implicit val a = x; /* more */ }
与往常一样，括号在其中开始一个新的作用域，我可以在其中引入新的val甚至是新的隐式.
我真的在做什么a?
我正在消耗堆栈空间吗?堆?还有别的别名吗?
像普通的val pat = expr定义一样，等号左侧的内容只是一个模式.
语法规范中的Enumerator生成显示，for-expr中的子句可以是生成器(a <- b)，guard if cond或val def a = b.
可以是任意表达式的部分是b(如<-和=的右侧所示)和条件.
Responder.exec利用条件来执行任意代码，同时对true进行微不足道的评估.
这意味着您可以根据条件进行任意副作用:
// yucky, yet instructive
scala> val xs = List(1,2,3)
scala> def bar(implicit i: Int) = Some(i+1)
scala> implicit var imp: Int = 0
scala> for { a<-xs; if { imp=a; true }; b<-bar } yield b
res6: List[Int] = List(2, 3, 4)
类似地，val def desugars如下:
tmp <- xs
a = f(tmp) // some arbitrary function of tmp
// amounts to
(tmp, a) <- for (x@tmp <- xs) yield { val x0@a=f(tmp); (x, x0) }
真的吗?
scala> def f(vs: List[Int]) = for (a <- vs; b = a+1) yield b
f: (vs: List[Int])List[Int]
您需要最近的代表才能这样做:
scala> :javap f
[snip]
public scala.collection.immutable.List<java.lang.Object> f(scala.collection.immutable.List<java.lang.Object>);
flags: ACC_PUBLIC
Code:
stack=3, locals=2, args_size=2
0: aload_1
1: new #16 // class $anonfun$f$1
4: dup
5: invokespecial #17 // Method $anonfun$f$1."<init>":()V
8: getstatic #22 // Field scala/collection/immutable/List$.MODULE$:Lscala/collection/immutable/List$;
11: invokevirtual #26 // Method scala/collection/immutable/List$.canBuildFrom:()Lscala/collection/generic/CanBuildFrom;
14: invokeinterface #32, 3 // InterfaceMethod scala/collection/TraversableLike.map:(Lscala/Function1;Lscala/collection/generic/CanBuildFrom;)Ljava/lang/Object;
19: checkcast #28 // class scala/collection/TraversableLike
22: new #34 // class $anonfun$f$2
25: dup
26: invokespecial #35 // Method $anonfun$f$2."<init>":()V
29: getstatic #22 // Field scala/collection/immutable/List$.MODULE$:Lscala/collection/immutable/List$;
32: invokevirtual #26 // Method scala/collection/immutable/List$.canBuildFrom:()Lscala/collection/generic/CanBuildFrom;
35: invokeinterface #32, 3 // InterfaceMethod scala/collection/TraversableLike.map:(Lscala/Function1;Lscala/collection/generic/CanBuildFrom;)Ljava/lang/Object;
40: checkcast #37 // class scala/collection/immutable/List
43: areturn
我看到了map的两个调用，分别是中间表达式和yield.
进一步检查时，第一个匿名提示不是Int => Int(即a+1)，而是Int => (Int,Int).
因此，我们引入的val只是作为元组的一部分被传递了.
When I use a val in a for-comprehension, I get the warning:
warning: val keyword in for comprehension is deprecated
despite the production in the syntax appendix of the spec.
This suggests that when I do something like
for (x <- xs; a = x)
I'm not really introducing a variable, such as if I do something like
for (x <- xs) yield { implicit val a = x; /* more */ }
where, as usual, the brace starts a new scope where I can introduce a new val, or even a new implicit.
What am I really doing with that a?
Am I consuming stack space? Heap? Some other kind of alias?
Like an ordinary val pat = expr definition, the thing to the left of the equals sign is just a pattern.
The Enumerator production in the syntax spec shows that the clause in a for-expr can be a generator (a <- b), guard if cond or val def a = b.
The parts that can be arbitrary expressions are b (as given to the right of <- and =) and the condition.
Responder.exec takes advantage of the conditional to execute arbitrary code, while evaluating trivially to true.
That means you could do arbitrary side-effects from a conditional:
// yucky, yet instructive
scala> val xs = List(1,2,3)
scala> def bar(implicit i: Int) = Some(i+1)
scala> implicit var imp: Int = 0
scala> for { a<-xs; if { imp=a; true }; b<-bar } yield b
res6: List[Int] = List(2, 3, 4)
Similarly, the val def desugars as follows:
tmp <- xs
a = f(tmp) // some arbitrary function of tmp
// amounts to
(tmp, a) <- for (x@tmp <- xs) yield { val x0@a=f(tmp); (x, x0) }
Wait, really?
scala> def f(vs: List[Int]) = for (a <- vs; b = a+1) yield b
f: (vs: List[Int])List[Int]
You'll need a recent repl to do this:
scala> :javap f
[snip]
public scala.collection.immutable.List<java.lang.Object> f(scala.collection.immutable.List<java.lang.Object>);
flags: ACC_PUBLIC
Code:
stack=3, locals=2, args_size=2
0: aload_1
1: new #16 // class $anonfun$f$1
4: dup
5: invokespecial #17 // Method $anonfun$f$1."<init>":()V
8: getstatic #22 // Field scala/collection/immutable/List$.MODULE$:Lscala/collection/immutable/List$;
11: invokevirtual #26 // Method scala/collection/immutable/List$.canBuildFrom:()Lscala/collection/generic/CanBuildFrom;
14: invokeinterface #32, 3 // InterfaceMethod scala/collection/TraversableLike.map:(Lscala/Function1;Lscala/collection/generic/CanBuildFrom;)Ljava/lang/Object;
19: checkcast #28 // class scala/collection/TraversableLike
22: new #34 // class $anonfun$f$2
25: dup
26: invokespecial #35 // Method $anonfun$f$2."<init>":()V
29: getstatic #22 // Field scala/collection/immutable/List$.MODULE$:Lscala/collection/immutable/List$;
32: invokevirtual #26 // Method scala/collection/immutable/List$.canBuildFrom:()Lscala/collection/generic/CanBuildFrom;
35: invokeinterface #32, 3 // InterfaceMethod scala/collection/TraversableLike.map:(Lscala/Function1;Lscala/collection/generic/CanBuildFrom;)Ljava/lang/Object;
40: checkcast #37 // class scala/collection/immutable/List
43: areturn
I see two invocations of map, for the intermediate expression and for the yield.
On further inspection, the first anonfun is not a Int => Int (i.e., a+1) but a Int => (Int,Int).
So the val we introduced is just getting passed around as part of a tuple.
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