通过 for-compression 线程“Try”

触发于另一个问题 https://stackoverflow.com/revisions/5cbd16fb-14e0-4870-aae7-b140601b1e3e/view-source（不过后来被编辑掉了），我想尝试一下链接调用 Scala 2.10 是多么容易Try构建（参见这个演示文稿 https://speakerdeck.com/u/heathermiller/p/futures-and-promises-in-scala-2-dot-10)，使用 for 推导式。

这个想法是拥有一个标记列表并将它们与一系列模式进行匹配，然后返回第一个错误或成功匹配的模式。我得到了以下相当尴尬的版本，我想知道是否可以使其变得更简单更好：

import util.Try

trait Token
case class Ident  (s: String) extends Token
case class Keyword(s: String) extends Token
case class Punct  (s: String) extends Token
case object NoToken extends Token
case class FunctionDef(id: Ident)

case class Expect[A](expectation: String)(pattern: PartialFunction[Token, A]) {
  def unapply(tup: (Try[_], Token)) = Some(tup._1.map { _ => 
     pattern.lift(tup._2).getOrElse(throw new Exception(expectation))
  })
}

现在构造期望Keyword("void") :: Ident(id) :: Punct("(") :: Punct(")") :: tail

val hasVoid   = Expect("function def starts with void") { case Keyword("void") => }
val hasIdent  = Expect("expected name of the function") { case id: Ident       => id }
val hasOpen   = Expect("expected opening parenthesis" ) { case Punct("(")      => }
val hasClosed = Expect("expected closing parenthesis" ) { case Punct(")")      => }

构建一个完整的测试用例：

def test(tokens: List[Token]) = {
  val iter = tokens.iterator
  def next(p: Try[_]) = Some(p -> (if (iter.hasNext) iter.next else NoToken))
  def first() = next(Try())

  val sq = for {
    hasVoid  (vd) <- first()
    hasIdent (id) <- next(vd)
    hasOpen  (op) <- next(id)
    hasClosed(cl) <- next(op)
  } yield cl.flatMap(_ => id).map(FunctionDef(_))

  sq.head
}

下面验证测试方法：

// the following fail with successive errors
test(Nil)
test(Keyword("hallo") :: Nil)
test(Keyword("void" ) :: Nil)
test(Keyword("void" ) :: Ident("name") :: Nil)
test(Keyword("void" ) :: Ident("name") :: Punct("(") :: Nil)
// this completes
test(Keyword("void" ) :: Ident("name") :: Punct("(") :: Punct(")") :: Nil)

现在特别是额外的flatMap and map in yield看起来很可怕，而且需要打电话head关于理解的结果。

有任何想法吗？是Try非常不适合理解？也不应该Either or Try被“固定”以允许这种类型的线程（例如允许Try作为直接结果类型unapply)?

诀窍似乎是not create Try中的实例inner结构，而是让它抛出异常并构造一个outer Try.

首先，让我们摆脱Try[Unit]'s:

case class Expect(expectation: String)(pattern: PartialFunction[Token, Unit]) {
  def unapply(token: Token) = 
    pattern.isDefinedAt(token) || (throw new Exception(expectation))
}

case class Extract[A](expectation: String)(pattern: PartialFunction[Token, A]) {
  def unapply(token: Token) = Some(
    pattern.lift(token).getOrElse(throw new Exception(expectation))
  )
}

然后检查就变成：

val hasVoid   = Expect ("function def starts with void") { case Keyword("void") => }
val getIdent  = Extract("expected name of the function") { case id: Ident       => id }
val hasOpen   = Expect ("expected opening parenthesis" ) { case Punct("(")      => }
val hasClosed = Expect ("expected closing parenthesis" ) { case Punct(")")      => }

以及测试方法：

def test(tokens: List[Token]) = Try {
  val iter = tokens.iterator
  def next() = Some(if (iter.hasNext) iter.next else NoToken)

  (for {
    hasVoid()    <- next()
    getIdent(id) <- next()
    hasOpen()    <- next()
    hasClosed()  <- next()
  } yield FunctionDef(id)).head  // can we get rid of the `head`?
}