如何在Scala中将此map/flatMap转换为for comprehension? [英] How to convert this map/flatMap into a for comprehension in Scala?

问题描述

如何将此map/flatMap转换为，以供理解，请解释其工作方式，谢谢.

How to convert this map/flatMap into a for comprehension, and please explain how it works, thanks.

    def compute2(maybeFoo: Option[Foo]): Option[Int] =
      maybeFoo.flatMap { foo =>
      foo.bar.flatMap { bar =>
          bar.baz.map { baz =>
          baz.compute
      }
    }
  }  

推荐答案

您的代码可以翻译为:

def compute2(maybeFoo: Option[Foo]): Option[Int] =
  for {
    foo <- maybeFoo
    bar <- foo.bar
    baz <- bar.baz
  } yield baz.compute

Scala编程第二版中的语录:

Quotes from Programming in Scala, Second Edition:

通常，for表达式的形式为:

Generally, a for expression is of the form:

for ( seq ) yield expr

在这里，seq是生成器，定义和过滤器的序列，在连续的元素之间带有分号.

Here, seq is a sequence of generators, definitions, and filters, with semi-colons between successive elements.

此表达式包含一个生成器，一个定义和一个过滤器:

This for expression contains one generator, one definition, and one filter:

for {
p <- persons // a generator
n = p.name // a definition
if (n startsWith "To") // a filter
} yield n

使用一个生成器翻译表达式

首先，假设您有一个简单的表达式:

First, assume you have a simple for expression:

for (x <- expr1) yield expr2

其中x是一个变量.这样的表达式被翻译为:

where x is a variable. Such an expression is translated to:

expr1.map(x => expr2)

翻译以生成器和过滤器开头的表达式

现在，考虑将前导生成器与某些生成器结合在一起的表达式 其他元素.用于表达的形式:

Now, consider for expressions that combine a leading generator with some other elements. A for expression of the form:

for (x <- expr1 if expr2) yield expr3

被翻译为:

expr1 withFilter (x => expr2) map (x => expr3)

翻译以两个生成器开头的表达式

下一个案例处理以两个生成器开头的表达式，如:

The next case handles for expressions that start with two generators, as in:

for (x <- expr1; y <- expr2) yield expr3

上面的for表达式被翻译为flatMap的应用程序:

The for expression above is translated to an application of flatMap:

expr1.flatMap(x => for (y <- expr2) yield expr3)

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