这个警告让我想不通"
[warn] -- [E198] Unused Symbol Warning: /Users/.../polycheck/helpers.scala:25:30
[warn] 25 |def hasStandardForm[P: PolyFamily](entry: P): Boolean = entry.asMatchable match ...
[warn] | ^
[warn] | unused implicit parameter虽然没有直接使用隐式参数，但它能阻止 hasStandardForm 接受不属于 PolyFamily 类型类的类型。这样使用一个（未使用的）隐式参数，不是很合理吗？
This warning has me puzzled"
[warn] -- [E198] Unused Symbol Warning: /Users/.../polycheck/helpers.scala:25:30
[warn] 25 |def hasStandardForm[P: PolyFamily](entry: P): Boolean = entry.asMatchable match ...
[warn] | ^
[warn] | unused implicit parameterAlthough the implicit isn't used directly, it prevents hasStandardForm from accepting types outside the PolyFamily typeclass. Doesn't that make an (unused) implicit perfectly legitimate here?
对，这就叫类型证据。
如果没记错，在 Scala 2 里命名为 ev 就不会警告，不过不清楚 Scala 3 是否也遵守这个约定。
或者直接加 @nowarn 就行。
Right, that's what people call evidence.
If memory serves, naming it ev avoids warnings in Scala 2, though I don't know whether Scala 3 honors that convention.
Alternatively, simply apply @nowarn.
叫 ev 还是有警告。
[warn] -- [E198] Unused Symbol Warning: /Users/tavin/work/exercises-853_962/5/PolyFamily/src/test/scala/polycheck/helpers.scala:25:34
[warn] 25 |def hasStandardForm[P](entry: P)(using ev: PolyFamily[P]): Boolean = entry.asMatchable match
[warn] | ^^
[warn] | unused implicit parameterCalling it ev still warns:
[warn] -- [E198] Unused Symbol Warning: /Users/tavin/work/exercises-853_962/5/PolyFamily/src/test/scala/polycheck/helpers.scala:25:34
[warn] 25 |def hasStandardForm[P](entry: P)(using ev: PolyFamily[P]): Boolean = entry.asMatchable match
[warn] | ^^
[warn] | unused implicit parameter有些常见类型不会触发这个警告，没有成员的类型（“标记接口”）也一样。
比如 Comparable 会触发警告。如果根本没比较任何东西，可能是约束已经没用了，也可能是忘记比较了。
scala> def inspect[A: Comparable] = "57".toInt
1 warning found
-- [E198] Unused Symbol Warning: -----------------------------------------------
1 |def inspect[A: Comparable] = "57".toInt
| ^
| unused implicit parameter
def inspect[A](using evidence$1: Comparable[A]): Int我觉得得在展开后的形式里明确标为未使用。
scala> def inspect[A](item: A)(using @unused witness: Comparable[A]) = item.toString
def inspect[A](item: A)(using witness: Comparable[A]): String这比 @nowarn 稍微清楚一点，不过后者能用于上下文界限的写法。
（我猜按名称检查是在可以选择下面选项之前的办法
-Wunused:explicits,implicits,synthetics
其实 Scala 3 目前还没有 synthetics，只有 Scala 2 有。
-Wunused:synthetics Warn if a synthetic implicit parameter (context bound) is unused.
-Wunused:nowarn Warn if a @nowarn annotation does not suppress any warnings.
-Wunused:params Enable -Wunused:explicits,implicits,synthetics.它是在疫情初期加进去的，大概是为了打发时间吧。）
Certain familiar types are exempt from this warning, as are memberless types (“marker interfaces”).
Comparable, for instance, triggers it. If nothing gets compared, perhaps the constraint has become unnecessary, or perhaps you neglected to make a comparison.
scala> def inspect[A: Comparable] = "57".toInt
1 warning found
-- [E198] Unused Symbol Warning: -----------------------------------------------
1 |def inspect[A: Comparable] = "57".toInt
| ^
| unused implicit parameter
def inspect[A](using evidence$1: Comparable[A]): IntI believe you have to mark it unused in this expanded version:
scala> def inspect[A](item: A)(using @unused witness: Comparable[A]) = item.toString
def inspect[A](item: A)(using witness: Comparable[A]): StringThat's somewhat cleaner than @nowarn, although the latter accommodates context-bound notation.
(I suspect name-based checks date to before selecting
-Wunused:explicits,implicits,synthetics
In fact, synthetics isn't available in Scala 3 yet, only Scala 2:
-Wunused:synthetics Warn if a synthetic implicit parameter (context bound) is unused.
-Wunused:nowarn Warn if a @nowarn annotation does not suppress any warnings.
-Wunused:params Enable -Wunused:explicits,implicits,synthetics.That arrived near the pandemic's beginning, presumably as a way of filling the hours.)
你好， 在一个我无法修改代码的包里，类的层次结构如下。 我想给 Inner 加个方便使用的包装，但因为 Inner 依赖外层实例，目前只做到把它放进外层类的子类里面。 可惜，在多个地方继承 Outer，就意味着每个新子类里都得重复这个方便使用的 ca…
我发现下面两个表达式 以及 行为完全相同。证明定理时也是这样。 它们可以互换吗？区别在哪里？
我在创建一个 Map，其中每个条目都保存字符串数组。怎样在一条语句中同时声明类型并完成初始化？ 我（天真地）这样试了一下。 结果是下面这个错误。
