for expression

How to use multiple Futures in a Scala for-comprehension alvin February 13, 2020 - 9:42am

If you want to create multiple Scala Futures and merge their results together to get a result in a for comprehension, the correct approach is to (a) first create the futures, (b) merge their results in a for comprehension, then (c) extract the result using onComplete or a similar technique.

Scala: How to use higher-order functions (HOFs) with Option (instead of match expressions) alvin February 13, 2020 - 9:40am

I originally wrote a long introduction to this article about how to work with the Scala Option, but I decided to keep that introduction for a future second article in this series. For this article I’ll just say:

  • idiomatic Scala code involves never using null values
  • because you never use nulls, it’s important for you to become an expert at using Option, Some, and None
  • initially you may want to use match expressions to handle Option values
  • as you become more proficient with Scala and Options, you’ll find that match expressions tend to be verbose
  • becoming proficient with higher-order functions (HOFs) like map, filter, fold, and many others are the cure for that verbosity

A small Scala 2 project converted to Dotty

If you want to see a somewhat larger example of Dotty source code than I’ve shown before, I just took a little time to convert a small Scala 2 project over to the new/current Dotty syntax (i.e., the Dotty syntax supported by the Dotty 0.21 release, circa January, 2020).

Scala: What do “effect” and “effectful” mean in functional programming?

When you get started with functional programming (FP) a common question you’ll have is, “What is an effect in functional programming?” You’ll hear advanced FPers use the words effects and effectful, but it can be hard to find a definition of what these terms mean.

Scala for/yield examples (for-loop and yield examples)

I just found some notes from when I first began working with Scala, and I was working with the yield keyword in for loops. If you haven't worked with something like yield before, it will be helpful to know how it works. Here's a statement of how the yield keyword works in for loops, based on the documentation in the book, Programming in Scala:

Scala: Examples of for-expressions being converted to map and flatMap

Without any explanation, here are a couple of source code examples from my book, Learning Functional Programming in Scala. The only thing I’ll say about this code is that I created it in the process of writing that book, and the examples show how the Scala compiler translates for-expressions into map and flatMap calls behind the scenes.

Scala: Handling nested Options with flatMap and for

Summary: In this article I show a couple of ways to extract information from optional fields in your Scala domain models. This example is a little contrived, but if you have a situation where one Option instance contains one or more other Options, this article may be helpful.

There are times when you’re creating your domain model when it makes sense to use optional fields in your case classes. For instance, when you model an Address, the “second street address” isn’t needed for all people, so making it an optional field makes sense:

This is a page from my book, Functional Programming, Simplified

How to Enable the Use of Multiple Generators in a Scala `for` Expression

One cool thing about for expressions is that you can use multiple generators inside of them. This lets you do some nice analytics when you have some interesting data relationships.

For instance, suppose you have some data like this:

This is a page from my book, Functional Programming, Simplified

How to Enable Filtering in a Scala `for` Expression

Next, let’s see if we can use a filtering clause inside of a for expression with the Sequence code we have so far.

Trying to use a filtering expression

When I paste the current Sequence class and this code into the Scala REPL:

val ints = Sequence(1,2,3,4,5)

val res = for {
    i <- ints
    if i > 2
} yield i*2

I see the following error message: