When is it appropriate to use an associated type versus a generic type?
In this question, an issue arose that could be solved by changing an attempt at using a generic type parameter into an associated type. That prompted the question "Why is an associated type more appropriate here?", which made me want to know more.
The RFC that introduced associated types says:
This RFC clarifies trait matching by:
- Treating all trait type parameters as input types, and
- Providing associated types, which are output types.
The RFC uses a graph structure as a motivating example, and this is also used in the documentation, but I'll admit to not fully appreciating the benefits of the associated type version over the type-parameterized version. The primary thing is that the distance method doesn't need to care about the Edge type. This is nice but seems a bit shallow of a reason for having associated types at all.
I've found associated types to be pretty intuitive to use in practice, but I find myself struggling when deciding where and when I should use them in my own API.
When writing code, when should I choose an associated type over a generic type parameter, and when should I do the opposite?
Answers
Deciding between using an associated type or a generic type parameter in Rust can sometimes be a nuanced decision, and the choice depends on various factors. Here are some guidelines to help you decide when to use each:
Associated Types:
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Clearer Trait Definitions: Associated types can make trait definitions cleaner and more focused by separating concerns. If a trait's implementation relies on a specific type that is intrinsic to the trait's functionality but is not known in advance, using an associated type can make the trait definition more intuitive.
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Implementation Flexibility: Associated types provide more flexibility in how trait implementations are structured. They allow implementors to choose concrete types for associated types independently for each implementation, which can be beneficial in certain scenarios.
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Default Implementations: Associated types can be used in combination with default implementations to provide default behavior for traits, allowing implementors to override specific associated types if needed.
Generic Type Parameters:
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Maximal Flexibility: Generic type parameters provide maximal flexibility as they allow traits to be defined without any assumptions about concrete types. This can be useful when a trait needs to operate on a wide range of types, and the exact types are not known in advance.
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Trait Bounds: Generic type parameters allow trait bounds to be specified directly on the function signatures, providing more control over the types that can be used with the trait. This can lead to clearer and more explicit code in some cases.
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Trait Object Compatibility: Generic type parameters are compatible with trait objects, whereas associated types are not. If you need to use trait objects, generic type parameters are the way to go.
When to Choose:
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Use Associated Types: When the trait's functionality is closely tied to a specific type that varies between implementations, and when you want to provide more clarity and structure to the trait definition.
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Use Generic Type Parameters: When the trait needs to operate on a wide range of types and the exact types are not known in advance, or when you need compatibility with trait objects.
In practice, you may find that certain traits are more naturally expressed with associated types, while others benefit from the flexibility provided by generic type parameters. It often comes down to the specific requirements and design goals of your API.
