Programming Scala, 3rd Edition

Chapter 8. for Comprehensions in Depth

“for Comprehensions” described the syntax for for comprehensions, including lots of examples. At this point, they look like a nice, more flexible version of the venerable for loop, but not much more. In fact, lots of sophistication lies below the surface. This chapter explores how for comprehension syntax is a more concise way to use foreach, map, flatMap, and withFilter, some of the functional combinators we discussed in the previous chapter. You can write concise code with elegant solutions to a number of design problems.

We’ll finish with some practical design problems implemented using for comprehensions, such as error handling during the execution of a sequence of processing steps.

Recap: The Elements of for Comprehensions

A for comprehension contains one or more generator expressions, optional guard expressions for filtering, and optional value definitions. The output can be yielded to create a new collection, or a side-effecting block of code can be executed on each pass, such as printing output. The following example demonstrates all these features. It removes blank lines from a text file. This is a full program with an example of how to parse input arguments (although there are libraries available for this purpose), handle help messages, etc.:

// src/main/scala/progscala3/forcomps/RemoveBlanks.scala
package progscala3.forcomps

object RemoveBlanks:
  def apply(path: String, compress: Boolean, numbers: Boolean): Seq[String] =
    for