Scalazzi safe Scala subset (Principles 2, Scalaz Files)

Scalaz is designed for programmers building type-safe, functional programs. If you program like this, you can start to see very deep properties of your functions by only reading their types; in other words, types become documentation. This also lets you see how you can combine your functions in more ways, with greater confidence that the combination will actually make sense.

But Scala, the language, contains many unsafe features that get in the way of this method of thinking about your programs. “Scalazzi” means that you are avoiding or banning these features from your Scala codebase, thus restoring your ability to use types to discover those properties.

1. null
2. exceptions
3. Type-casing (isInstanceOf)
4. Type-casting (asInstanceOf)
5. Side-effects
6. equals/toString/hashCode
7. notify/wait
8. classOf/.getClass
9. General recursion

Here’s an example of how you might use these rules to reduce your testing requirements.

Suppose that you have this very simple function to return the greater Int.

def maximum(x: Int, y: Int): Int = if (x >= y) x else y

(I encourage you to imagine that this is harder than this example; after all, aren’t your own programs more complicated?) This type signature says that any Int can be returned; we must test to verify that this isn’t happening.

Instead of writing a test, we can use parametricity to check that either x or y is returned, but nothing else, at compile time instead of test time.

def maximum[N <: Int](x: N, y: N): N = // • • •

I can read from this type that only x or y can be returned. With some practice, you’ll start to see more complex facts arising from types as well.

Unfortunately, Scala has many “features” that let you break this safety. These features aren’t useful for type-safe functional programs, so we simply declare them verboten.

Scalaz expects you to follow Scalazzi rules, but is also packed with features to help you follow them. For example, if you are calling map on a List and feel like your lambda needs to perform some side effect, it’s time to look into Scalaz’s Traverse typeclass.

A standard library for principled functional programming in Scala (Principles 1, Scalaz Files)

The best way to think about “what is Scalaz?” is as a standard library for functional programming. This goes all the way back to its creation: the Scalaz project started because there are not enough facilities in Scala's standard library for convenient, everyday functional programming, without cheating.

How should this affect your approach to the library? Like a standard library, you learn the bits and pieces you need, not the whole thing. There is no must-read book, no must-watch tutorial video, no must-attend course. Scalaz can be used successfully from day 1 as a new Scala programmer; as you do not learn every part of the standard library before starting to use a language, so it goes for Scalaz as well. All that is required of you is the desire to solve programming problems in type-safe, functional ways, and the curiosity to learn about what components that others have discovered and how they might be useful. After all, most pieces of Scalaz were added to it because somebody was solving a problem, and found a solution they thought others might consider useful and well-thought-out.