Effect System

Note: The following text applies to Flix 0.54.0 or later.

Flix features a state-of-the-art type and effect system fully integrated into the language. The Flix effect system is powerful and extensive, supporting effect polymorphism, sub-effecting, effect exclusion, purity reflection, and associated effects.

We will explore these new and exciting features over the coming pages.

What are the benefits of an effect system? There are many:

• (Purity) A type and effect system separates pure and impure functions. In Flix, a pure function cannot have any side-effects and must return the same value when given the same arguments. Nevertheless, a pure function can still be implemented in an imperative style using mutable data structures as long as those data structures leave scope when the function ends.

• (Reasoning) A type and effect system helps programmers understand how their programs work by requiring every function to specify its argument and return types, as well as the side-effects of the function.

• (Modularity) A type and effect system enforces modularity by forcing programmers to consider what side effects are allowed where in the program. Moreover, effects — like types — serve as compiler checked documentation.

• (Effects and Handlers) A type and effect system is the foundation for algebraic effects and handlers. These allow programmers to implement their own control structures, such as exceptions, async/await, and cooperative multitasking.

• (Security) A type and effect system offers iron-clad guarantees about the behavior of functions, allowing programmers to increase their trust in unknown code. For example, if a function is pure, it cannot have any side-effects: it cannot access the file system, the network, etc. A specific benefit is that programs become more resistant to supply chain attacks.

• (Purity Reflection) The Flix Standard Library (and other library authors in extension) can use purity reflection to inspect the purity of function arguments passed to higher-order functions. We can exploit this information to implement automatic parallelization while preserving the original semantics of the program. For example, in Flix, the Set.count function uses parallel evaluation if (a) the set is sufficiently large and (b) the passed predicate function is pure.

• (Optimizations) The Flix compiler exploits purity information for aggressive dead code elimination and inlining.

The Flix type and effect system is quite sophisticated and requires some background knowledge to use effectively. In the next couple of sections, we gradually introduce the features of the type and effect system and give several examples of its use.

Before we continue, it is important to understand that Flix has three types of effects:

• foundational effects
• algebraic effects
• heap effects

We describe how traits and effects interact in the section on Associated Effects.

Direct Style

Flix is a so-called direct-style programming language with a traditional type and effect system. This is in contrast to so-called functional effect systems like Cats Effect, Kyo, and ZIO. These systems offer a library-level effect system, essentially a custom IO monad. While this approach has some advantages, the downsides are at least twofold (a) we do not get any of the guarantees offered by a traditional type and effect system (e.g., we cannot know when a function is pure), and (b) we must write our program in a monadic-style which is burdensome.