我有这样的事情：
case class Box[A](x: A) {
def flatMap[B](f: A => GenTraversableOnce[B]): GenTraversableOnce[B] =
f(x)
def flatMap[B](f: A => Box[B]): Box[B] =
f(x)
def map[B](f: A => B): Box[B] =
Box(f(x))
}
object Box {
for {
i <- Box(0)
j <- Box(1)
} yield i + j
}
上面的代码在Scala 2.12.4中编译得很好，但是不能在Scala 2.11.12中编译：
[error] Box.scala:10: missing parameter type
[error] i <- Box(0)
[error] ^
[error] one error found
[error] (core/compile:compileIncremental) Compilation failed
为什么呢？我做错了什么？
然后我尝试了：
for {
i: Int <- Box(0)
j: Int <- Box(1)
} yield i + j
现在令人惊讶的是，代码在Scala 2.11.12中编译，但不能在Scala 2.12.4中编译！？！
[error] Box.scala:10: pattern var i in value $anonfun is never used; `i@_' suppresses this warning
[error] i: Int <- Box(0)
[error] ^
[error] Box.scala:11: pattern var j in value $anonfun is never used; `j@_' suppresses this warning
[error] j: Int <- Box(1)
[error] ^
[error] two errors found
[error] (core/compile:compileIncremental) Compilation failed
这些是我正在使用的scalac标志：
def scalacOptionsForVersion(scalaVersion: String) = CrossVersion.partialVersion(scalaVersion) match {
case Some((2, 10)) => Seq(
"-deprecation",
"-encoding", "UTF-8",
"-feature",
"-language:implicitConversions",
"-language:reflectiveCalls",
"-unchecked",
"-Xfatal-warnings",
"-Xlint",
"-Yinline-warnings",
"-Yno-adapted-args",
"-Ywarn-dead-code",
//"-Ywarn-numeric-widen", // bugs in 2.10
"-Xfuture"
)
case Some((2, 11)) => Seq( // Copied from https://tpolecat.github.io/2014/04/11/scalac-flags.html
"-deprecation",
"-encoding", "UTF-8", // yes, this is 2 args
"-feature",
"-language:existentials",
"-language:higherKinds",
"-language:implicitConversions",
"-unchecked",
"-Xfatal-warnings",
"-Xlint",
"-Yno-adapted-args",
"-Ywarn-dead-code", // N.B. doesn't work well with the ??? hole
"-Ywarn-numeric-widen",
//"-Ywarn-value-discard", // This is broken in 2.11 for Unit types
"-Xfuture",
"-Ywarn-unused-import" // 2.11 only
)
case Some((2, 12)) => Seq( // Copied from https://tpolecat.github.io/2017/04/25/scalac-flags.html
"-deprecation", // Emit warning and location for usages of deprecated APIs.
"-encoding", "utf-8", // Specify character encoding used by source files.
"-explaintypes", // Explain type errors in more detail.
"-feature", // Emit warning and location for usages of features that should be imported explicitly.
"-language:existentials", // Existential types (besides wildcard types) can be written and inferred
"-language:experimental.macros", // Allow macro definition (besides implementation and application)
"-language:higherKinds", // Allow higher-kinded types
"-language:implicitConversions", // Allow definition of implicit functions called views
"-unchecked", // Enable additional warnings where generated code depends on assumptions.
"-Xcheckinit", // Wrap field accessors to throw an exception on uninitialized access.
"-Xfatal-warnings", // Fail the compilation if there are any warnings.
"-Xfuture", // Turn on future language features.
"-Xlint:adapted-args", // Warn if an argument list is modified to match the receiver.
"-Xlint:by-name-right-associative", // By-name parameter of right associative operator.
"-Xlint:constant", // Evaluation of a constant arithmetic expression results in an error.
"-Xlint:delayedinit-select", // Selecting member of DelayedInit.
"-Xlint:doc-detached", // A Scaladoc comment appears to be detached from its element.
"-Xlint:inaccessible", // Warn about inaccessible types in method signatures.
"-Xlint:infer-any", // Warn when a type argument is inferred to be `Any`.
"-Xlint:missing-interpolator", // A string literal appears to be missing an interpolator id.
"-Xlint:nullary-override", // Warn when non-nullary `def f()' overrides nullary `def f'.
"-Xlint:nullary-unit", // Warn when nullary methods return Unit.
"-Xlint:option-implicit", // Option.apply used implicit view.
"-Xlint:package-object-classes", // Class or object defined in package object.
"-Xlint:poly-implicit-overload", // Parameterized overloaded implicit methods are not visible as view bounds.
"-Xlint:private-shadow", // A private field (or class parameter) shadows a superclass field.
"-Xlint:stars-align", // Pattern sequence wildcard must align with sequence component.
"-Xlint:type-parameter-shadow", // A local type parameter shadows a type already in scope.
"-Xlint:unsound-match", // Pattern match may not be typesafe.
"-Yno-adapted-args", // Do not adapt an argument list (either by inserting () or creating a tuple) to match the receiver.
"-Ypartial-unification", // Enable partial unification in type constructor inference
"-Ywarn-dead-code", // Warn when dead code is identified.
"-Ywarn-extra-implicit", // Warn when more than one implicit parameter section is defined.
"-Ywarn-inaccessible", // Warn about inaccessible types in method signatures.
"-Ywarn-infer-any", // Warn when a type argument is inferred to be `Any`.
"-Ywarn-nullary-override", // Warn when non-nullary `def f()' overrides nullary `def f'.
"-Ywarn-nullary-unit", // Warn when nullary methods return Unit.
"-Ywarn-numeric-widen", // Warn when numerics are widened.
"-Ywarn-unused:implicits", // Warn if an implicit parameter is unused.
"-Ywarn-unused:imports", // Warn if an import selector is not referenced.
"-Ywarn-unused:locals", // Warn if a local definition is unused.
"-Ywarn-unused:params", // Warn if a value parameter is unused.
"-Ywarn-unused:patvars", // Warn if a variable bound in a pattern is unused.
"-Ywarn-unused:privates", // Warn if a private member is unused.
"-Ywarn-value-discard" // Warn when non-Unit expression results are unused.
)
case _ => throw new IllegalArgumentException(s"No scalacOptions found for Scala $scalaVersion")
}
如何编写此代码，以便在Scala 2.11.x和2.12.x中编译而不需要显式类型？
答案 0 :(得分：5)
它并不完美，但你可以使用基于implicits的解决方案而不是重载。
import scala.collection.GenTraversableOnce
case class Box[A](x: A) {
def flatMap[B, F[_]](f: A => F[B])(implicit fmap: Box.FlatMap[F]): F[B] =
fmap(x)(f)
def map[B](f: A => B) = Box(f(x))
}
object Box {
sealed trait FlatMap[F[_]] {
def apply[A, B](a: A)(f: A => F[B]): F[B]
}
implicit val GenTraversableOnceFM = new FlatMap[GenTraversableOnce] {
def apply[A, B](a: A)(f: A => GenTraversableOnce[B]) = f(a)
}
implicit val BoxFM = new FlatMap[Box] {
def apply[A, B](a: A)(f: A => Box[B]) = f(a)
}
}