For-comprehensions and monads in Scala
Problem
At first glance, Scala for-comprehensions look like common for-loops.In the following example we want to create an address, but only if street, number and zipCode are available.
val maybeStreet: Option[String] = ???
val maybeNumber: Option[Int] = ???
val maybeZipCode: Option[Int] = ???
case class Address(street: String, number: Int, zipCode: Int)The Option[A] type in Scala has two forms, Some[A] and None.We could use flatMap and map in order to implement it.
val maybeAddress: Option[Address] = maybeStreet.flatMap(street =>
maybeNumber.flatMap(number =>
maybeZipCode.map(zipCode => Address(street, number, zipCode))))Solution
We could alternatively solve this with a for-comprehension.
val address: Option[Address] = for {
street <- maybeStreet
number <- maybeNumber
zipCode <- maybeZipCode
} yield Address(street, number, zipCode)The second implementation is more compact and readable.Only when all street and number and zipCode are instances of Some, we get Some[Address] as result. Otherwise, the result is None.
Further Aspects
In such a nested for-comprehension with yield, any data type that implements the functions map and flatMap can be used. For example, all monads would be possible. If you are wondering now what a monad is: The answer to this question would go beyond the scope of this ToiletPaper.But one thing is for sure: With for-comprehensions and monads, operations can be chained, which are to be executed one after the other. While doing so, an operation can use the output of previous operations as input. The concrete implementation of the monad defines what exactly happens at the boundary between the operations.
The behavior of a for-comprehension depends on which monad is used. Other commonly used monads in Scala include List[A], Either[A, B], Try[A], and Future[A].
For lists, the behavior is similar to iteration.
val result: List[(Int, String)] = for {
a <- List(1, 2, 3)
b <- List("A", "B", "C")
} yield (a, b)
// Result: List((1, "A"), (1, "B"), "....
---
Author: Aleksandar Stoimenov
Download Toilet Paper #157: For-comprehensions and monads in Scala (PDF)
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