Chapter 43: For Comprehensions

This lesson is going to explain for-expressions. Very excited - these have been a challenging part of the language specification for me to understand in day to day work.

The author starts by mentioning that a for-comprehension is another way to use a for loop. Yeah, this has been confusing me - I know:

def stackManip: State[Stack, Int] = for {
  _ <- push(3)
  a <- pop
  b <- pop
} yield(b)

Terminology why, let’ no longer refer to this as a for-loop - it’s not a for-comprehension. It’s still doing a loop but its actually an iteration. (lol, what’s the difference).

For comprehensions have the form for (enumerators) yield x where the enumerators either introduce new variables or are a filter. This is similar to a set-comprehension, and gives you a way to filter, transform, and combine lists.

A for-comprehension can contain the following expressions:

  • Generators
  • Filters
  • Definitions Example:
    for {
    p <- persons //generator
    n = // definition
    if(n startsWith "What?") //filter
    } yield


General form of pattern <- expression. Where pattern iterates all of the results from expression. for-comprehension begins with a generator.


General form of pattern = expression. Binds pattern to the value returned from expression. Same effect as writing val pattern = expression out of the for comprehension.


General form if (expression), which drops all elements from the iteration which the expression returns false.

Long Example:

case class Person(firstName: String, lastName: String)

val people = List(
  Person("Chris", "Paika")
  Person("Chris2", "Paika")

val namesStartingWithC = for {
  person <- people
  name = person.firstName
  if(name startsWith "C")  
} yield name.toUpperCase

I promise - I wrote that without peaking at the solution!

Chapter 44: Writing a class for a for-comprehension

This chapter’s short so I’m going to stick with it.

For-Comprehensions get converted by the compiler into a sequence of:

  • map
  • flatMap
  • foreach
  • withFilter

So as long as you have a class that implements those four methods, it can be used inside of a for comprehension. Technically you can implement a subset of them and have an access to a subset of a for-comprehensions ability, but that’s just complicated.