scala - 如何将此理解转化为 flatMap 实现

Question

我试图将此理解转化为 flatMap/map 实现，但我正在努力弄清楚如何:

def operationParser: Parser[(Operation, Int, Int)] = {
    for {
      n1 <- Parser.natural
      _ <- Parser.list(Parser.space)
      op <- Parser.operation
      _ <- Parser.list(Parser.space)
      n2 <- Parser.natural
    } yield (op, n1, n2)
  }

Answer 1

您可以使用scalac 的-Xprint:parser 命令行选项来查看脱糖版本:

源代码

object Main {
  trait Operation

  trait Parser[A] {
    def map[B](fn: A => B): Parser[B] = ???
    def flatMap[B](fn: A => Parser[B]): Parser[B] = ???
  }

  object Parser {
    def natural: Parser[Int] = ???
    def space: Parser[String] = ???
    def list[A](p: Parser[A]): Parser[A] = ???
    def operation: Parser[Operation] = ???
  }

  def operationParser: Parser[(Operation, Int, Int)] = {
    for {
      n1 <- Parser.natural
      _ <- Parser.list(Parser.space)
      op <- Parser.operation
      _ <- Parser.list(Parser.space)
      n2 <- Parser.natural
    } yield (op, n1, n2)
  }
}

编译器命令和输出

$ scalac -Xprint:parser Main.scala 
[[syntax trees at end of                    parser]] // Main.scala
package <empty> {
  object Main extends scala.AnyRef {
    def <init>() = {
      super.<init>();
      ()
    };
    abstract trait Operation extends scala.AnyRef;
    abstract trait Parser[A] extends scala.AnyRef {
      def $init$() = {
        ()
      };
      def map[B](fn: _root_.scala.Function1[A, B]): Parser[B] = $qmark$qmark$qmark;
      def flatMap[B](fn: _root_.scala.Function1[A, Parser[B]]): Parser[B] = $qmark$qmark$qmark
    };
    object Parser extends scala.AnyRef {
      def <init>() = {
        super.<init>();
        ()
      };
      def natural: Parser[Int] = $qmark$qmark$qmark;
      def space: Parser[String] = $qmark$qmark$qmark;
      def list[A](p: Parser[A]): Parser[A] = $qmark$qmark$qmark;
      def operation: Parser[Operation] = $qmark$qmark$qmark
    };
    def operationParser: Parser[scala.Tuple3[Operation, Int, Int]] = Parser.natural.flatMap(((n1) => Parser.list(Parser.space).flatMap(((_) => Parser.operation.flatMap(((op) => Parser.list(Parser.space).flatMap(((_) => Parser.natural.map(((n2) => scala.Tuple3(op, n1, n2)))))))))))
  }
}

经过一些格式化后，脱糖版本如下所示:

def operationParser: Parser[scala.Tuple3[Operation, Int, Int]] =
  Parser.natural.flatMap(((n1) =>
    Parser.list(Parser.space).flatMap(((_) =>
      Parser.operation.flatMap(((op) =>
        Parser.list(Parser.space).flatMap(((_) =>
          Parser.natural.map(((n2) =>
            scala.Tuple3(op, n1, n2)))))))))))

您可以使用此命令查看可用编译阶段的列表:

$ scalac -Xshow-phases
    phase name  id  description
    ----------  --  -----------
        parser   1  parse source into ASTs, perform simple desugaring
         namer   2  resolve names, attach symbols to named trees
packageobjects   3  load package objects
         typer   4  the meat and potatoes: type the trees
        patmat   5  translate match expressions
superaccessors   6  add super accessors in traits and nested classes
    extmethods   7  add extension methods for inline classes
       pickler   8  serialize symbol tables
     refchecks   9  reference/override checking, translate nested objects
       uncurry  10  uncurry, translate function values to anonymous classes
     tailcalls  11  replace tail calls by jumps
    specialize  12  @specialized-driven class and method specialization
 explicitouter  13  this refs to outer pointers
       erasure  14  erase types, add interfaces for traits
   posterasure  15  clean up erased inline classes
      lazyvals  16  allocate bitmaps, translate lazy vals into lazified defs
    lambdalift  17  move nested functions to top level
  constructors  18  move field definitions into constructors
       flatten  19  eliminate inner classes
         mixin  20  mixin composition
       cleanup  21  platform-specific cleanups, generate reflective calls
    delambdafy  22  remove lambdas
         icode  23  generate portable intermediate code
           jvm  24  generate JVM bytecode
      terminal  25  the last phase during a compilation run