← Previous in this series: Effects

In the previous chapter we talked about effects, how to use them, and what are they useful for. We chose one of the available effect types, and saw that we can do all kinds of things with it. We can map or flatmap over it, and this is what makes them composable, otherwise their usefulness would be very limited. We can use it to suspend side effects. We can use it to lift pure computations into the effect type so that we can compose those computations with effectful ones. We can even use it to run the so called “unsafe” operations and force the side effects to happen making our code not pure.

On one hand, our effect type is extremely powerful and often allows us to do a lot more than we need, or even desire to make possible in certain parts of the code. On the other hand, it is also “closed”, in the functionality that it carries with it. In the case of our example, we were stuck with whatever functionality cats.effect.IO provides, and that’s it.

Principle of the Least Powerful Abstraction

The principle of the least powerful abstraction says that your code should lean on the least powerful abstraction possible that it requires, in order to be able to do what it needs. So, for example, if we have an IO[String] and we want to perform a computation on it that will return an IO[Int], we only need an abstraction that provides us with map. Yet we have a concrete implementation, IO, that provides us not only with map but also a lot of other powerful tools that we don’t intend to either use, or allow others to use (for example the unsafe* methods of IO). That means we’re violating the principle of the least powerful abstraction by giving this piece of code a lot more power than it needs to.

There are several benefits to constraining the power of an abstraction in a specific scope. The first one being that it makes intentions explicit, if I write a function that takes a Functor, you know by this information alone that it can only map, because Functor is an abstraction to which the information you have about it, is that it provides map. If I write one that takes an IO, you can tell nothing about it, because IO is a specific implementation that allows you to do anything. The other major benefit is that it also leaves less scope for us to shoot ourselves in the foot, using the Functor example again, you can’t run any of the unsafe* methods on Functor but you can with IO.

An example that might ring with more developers is transforming a list using map vs transforming a list using a foreach loop. Using map, the only thing you can to is apply a function to each element of the list, and you will get a list of the same length with the result of the function applied to each element of that list. foreach on the other hand, is infinitely more powerful, there’s virtually no limits to what you can do with it, leaving you in a situation where it’s not immediately clear what the code does, and where there’s a lot more room to introduce bugs.

Type classes

So where does this leaves us? Why am I comparing IO with something called Functor and what is even this Functor that we’re talking about? ¹

We’ve talked about type parameters before, and at the time, we kept them completely generic, for example, we mentioned Option[A]. What do we know about A? We know absolutely nothing about it, right? Let’s say we were implementing map for Option, what could we do with value of type A? Since we know nothing about A, we certainly can’t call methods on it, and since map takes a function from A -> B, that’s basically all that we can do, apply that function to the value of type A to get a B. So, A is completely polymorphic, it can be replaced by any type, but that also means that there’s very little you can do with it (not necessarily a bad thing though).

Is there a way we can have a type parameter and know something about it, then? Well, yes, that’s where type classes come in. Type classes are another way to achieve polymorphism, also known as late binding ad hoc polymorphism, in contrast to parametric polymorphism (our A in Option), and early binding ad hoc polymorphism (inheritance). In Haskell you define a type class using the keyword class, you can then define type class instances with the keyword instance or, if it’s possible to derive the type class for a type, using the deriving keyword. In Scala declaring type classes is quite a ugly workaround. ²

The intuition for a type class can be, in my opinion, better explained by comparison to inheritance. In inheritance, a type is polymorphic by introducing a “is a” relationship. You create a class (in the OO sense of the word, not in the type class sense of the word), and during its declaration you say that this class is a something. Ex: class Cat extends Mammal, you say Cat is a Mammal. This is early binding polymorphism, as the relationship is defined at declaration time, once the class has been declared, you can’t say that it is anything more. With type classes the relationship is “has a”, so it’s late binding, the type is defined without knowledge of the type class, and an instance of the type class is provided at any point, and as long as there’s an instance in scope (the type has a), then the functionality provided by the type class is available for this type. So if there is a type class Mammal, and a type Cat, in Scala you would declare an instance of Mammal[Cat] and having it in scope means that Cat has a Mammal instance.

Back to the above example of Functor and IO, Functor is a type class, and, in the cats ecosystem, there is an instance of Functor for IO.

Type Class Constraints

We talked about type parameters and type classes, but we haven’t looked into how they’re used together yet. Let’s do that now.

If we have a type class MyTypeClass, and we have some type MyType parameterised on A, MyType[A], we can constrain A to be a type that has to have an instance of MyTypeClass available in the scope where MyType is used by declaring it like this MyType[A: MyTypeClasss]. And now, we know that A supports whatever functionality MyTypeClass provides. So A is less polymorphic that the A in Option, but it also means that by making it less polymorphic, there’s more we know about it and more we can do with it ³.

We’ve talked before about how type constructors are functions, and that some types have kind * -> *, also known as higher kinded types. IO is one such type constructor. In Scala syntax, this presents us with a problem, when trying to define a type parameter where we can “fit” IO. If we look at MyType[A], we see that MyType is a type constructor that expects a type to take the place of the parameter A to make a concrete type. Meaning we couldn’t pass IO as a parameter to MyType, because IO is not a concrete type, but rather a type constructor (we could pass IO[String] for example, as this is a concrete type, but that’s not what we want). To work around this, Scala allows us to define higher kinded type parameters with the syntax MyType[A[_]], although by convention we wouldn’t use A but rather F. Now, the MyType type constructor requires a higher kinded type as a parameter.

So, if we have MyType[F[_]], we can now constrain it to a type class, like Functor, so that we end up with something like MyType[F[_]: Functor]. Now, since IO is a higher kinded type, and it has an instance of Functor, we can pass IO to MyType. Inside of the abstraction encapsulated by MyType, we don’t know that it is being used with IO, and it might as well be used with something else, and we are now also limited to calling the functionality of Functor inside of it, so we can’t suspend side effects or run the unsafe methods on it. We also know these things just by looking at the signature of MyType and we can be sure that it won’t do more than map over our IO (or whatever other type with an instance of Functor we pass to it).

A breakdown of the type class hierarchy in cats can be found in the documentation, here and the most common ones to depend on when dealing with effect types will likely be Functor, Applicative, ApplicativeError, Monad, MonadError, and Sync, in order from the least to the most powerful.

F[_]

This is what it means when people say that they write an application in F, it means that at the entry point of your application you instantiate a concrete effect type, but from that point on, everything relies on a less powerful abstraction, preferably an unconstrained F[_], but more often than not, an F[_] that requires one of the above mentioned type classes.

Conclusion

In the beginning we’ve mentioned that by tying ourselves to IO, we were also limiting the functionality that we had at our disposal, and yet none of what we talked about in here addresses that. Because this chapter is getting a bit long, we’re going to address this in the next chapter.

In the next chapter we’re going to discuss advantages and disadvantages of adding functionality to F[_] using MTL style and type classes.