线程“for‘s through for-comprehension”

Question

由another question触发(不过后来被编辑掉了)，我想尝试一下将调用链接到Scala2.10的Try结构是多么容易(参见：this presentation)，使用for-comprehensions。

其思想是有一个令牌列表，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。我得到了下面这个相当尴尬的版本，我想知道是否可以让它变得更简单和更好：

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))
  })
}

现在构建对Keyword("void") :: Ident(id) :: Punct("(") :: Punct(")") :: tail的期望

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(")")      => }

构造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
}

以下是对测试方法的验证：

// 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)

现在尤其是yield中额外的flatMap和map看起来很可怕，以及需要对结果调用head来进行理解。

有什么想法吗？Try非常不适合用于理解吗？Either或Try是否应该被“修复”以允许这种类型的线程(例如，允许Try作为unapply的直接结果类型)？

Answer 1

诀窍似乎不是在内部结构中创建Try实例，而是让它抛出异常并构造一个外部Try。

首先，让我们去掉Try[Unit]：

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))
  )
}

那么检查就变成了：

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`?
}