scala 为什么我会在一种情况下而不是另一种情况下得到“缺少扩展函数的参数”？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/4701761/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Why do I get a "missing parameter for expanded function" in one case and not the other?
提问by ebruchez
Case this works:
案例这有效：
Seq(fromDir, toDir) find (!_.isDirectory) foreach (println(_))
Whereas this doesn't:
而这不会：
Seq(fromDir, toDir) find (!_.isDirectory) foreach (throw new Exception(_.toString))
Compilation ends with this error:
编译以这个错误结束：
error: missing parameter type for expanded function ((x) => x.toString)
Now if I write it this way it compiles again:
现在，如果我这样写，它会再次编译：
Seq(fromDir, toDir) find (!_.isDirectory) foreach (s => throw new Exception(s.toString))
I am sure there is a reasonable explanation ;)
我相信有一个合理的解释;)
回答by Francois G
This has already been addressed in a related question. Underscores extend outwards to the closest closing Expr: top-level expressions or expressions in parentheses.
这已经在相关问题中得到解决。下划线向外延伸到最接近的结尾Expr：顶级表达式或括号中的表达式。
(_.toString)is an expression in parentheses. The argument you are passing to Exceptionin the error case is therefore, after expansion, the full anonymous function (x$1) => x$1.toStringof type A <: Any => String, while Exceptionexpects a String.
(_.toString)是括号中的表达式。Exception因此，您在错误情况下传递给的参数是，在扩展之后，(x$1) => x$1.toStringtype的完整匿名函数A <: Any => String，而Exception期望 a String。
In the printlncase, _by itself isn't of syntactic category Expr, but (println (_))is, so you get the expected (x$0) => println(x$0).
在这种println情况下，_本身不是语法 category Expr，而是(println (_))，所以你得到了预期的(x$0) => println(x$0).
回答by Daniel C. Sobral
The difference is whether _stands for the whole parameter, or is part of an expression. Depending on which, it falls into one of the two following categories:
区别在于是_代表整个参数，还是表达式的一部分。根据哪个，它属于以下两个类别之一：
Partially Applied Function
部分应用函数
Seq(fromDir, toDir) find (!_.isDirectory) foreach (println(_))
translates into
翻译成
Seq(fromDir, toDir) find (!_.isDirectory) foreach ((x) => println(x))
Anonymous Function Parameter
匿名函数参数
Seq(fromDir, toDir) find (!_.isDirectory) foreach (throw new Exception(_.toString))
translates into
翻译成
Seq(fromDir, toDir) find (!_.isDirectory) foreach (throw new Exception((x) => x.toString))
