为什么我必须明确声明 Tuple2(a, b) 才能在 FoldLeft 中使用 Map add ？
我希望创建一个按名称键控的地图，其中包含具有该名称的事物的计数。我有一个带有名称的事物列表，其中可能包含多个具有相同名称的项目。像这样编码，我得到一个错误“类型不匹配；发现：需要字符串：(String, Int)”：
//variation 0, produces error
(Map[String, Int]() /: entries)((r, c) => { r + (c.name, if (r.contains(c.name)) (c.name) + 1 else 1) })
这让我很困惑，因为我认为 (a, b) 是一个 Tuple2，因此适合与 Map add 一起使用。以下任一变体均按预期工作：
//variation 1, works
(Map[String, Int]() /: entries)((r, c) => { r + Tuple2(c.name, if (r.contains(c.name)) (c.name) + 1 else 1) })
//variation 2, works
(Map[String, Int]() /: entries)((r, c) => {
val e = (c.name, if (r.contains(c.name)) (c.name) + 1 else 1) })
r + e
我不清楚为什么我的第一个版本有问题；任何人都可以建议。我正在使用 Scala-IDE 2.0.0 beta 2 来编辑源代码；该错误来自 Eclipse Problems 窗口。
I wish to create a Map keyed by name containing the count of things with that name. I have a list of the things with name, which may contain more than one item with the same name. Coded like this I get an error "type mismatch; found : String required: (String, Int)":
//variation 0, produces error
(Map[String, Int]() /: entries)((r, c) => { r + (c.name, if (r.contains(c.name)) (c.name) + 1 else 1) })
This confuses me as I though (a, b) was a Tuple2 and therefore suitable for use with Map add. Either of the following variations works as expected:
//variation 1, works
(Map[String, Int]() /: entries)((r, c) => { r + Tuple2(c.name, if (r.contains(c.name)) (c.name) + 1 else 1) })
//variation 2, works
(Map[String, Int]() /: entries)((r, c) => {
val e = (c.name, if (r.contains(c.name)) (c.name) + 1 else 1) })
r + e
I'm unclear on why there is a problem with my first version; can anyone advise. I am using Scala-IDE 2.0.0 beta 2 to edit the source; the error is from the Eclipse Problems window.
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当将单个元组参数传递给与运算符表示法一起使用的方法（例如
+方法）时，您应该使用双括号：我还更改了
Int的计算，在你的例子中看起来很有趣......When passing a single tuple argument to a method used with operator notation, like your
+method, you should use double parentheses:I've also changed the computation of the
Int, which looks funny in your example…因为
+用于连接字符串和字符串。在这种情况下，括号并不表示元组，而是表示参数。Scala 使用
+来做其他事情，这导致了各种各样的问题，就像你提到的那样。将
+替换为updated，或使用->代替,。Because
+is used to concatenate strings stuff with strings. In this case, parenthesis are not being taken to mean a tuple, but to mean a parameter.Scala has used
+for other stuff, which resulted in all sorts of problems, just like the one you mention.Replace
+withupdated, or use->instead of,.被解析为
因此编译器在
Map上查找 带有 2 个参数 的+方法，但没有找到。相对于双括号（正如 Jean-Philippe Pellet 所建议的那样），我更喜欢的形式是UPDATE：
如果 Pellet 是正确的，那么最好这样写
（当然 James Iry 的解决方案更好地表达了相同的意图）。
is parsed as
So the compiler looks for a
+method with 2 arguments onMapand doesn't find it. The form I prefer over double parentheses (as Jean-Philippe Pellet suggests) isUPDATE:
if Pellet is correct, it's better to write
(and of course James Iry's solution expresses the same intent even better).