● Covered by 1 source · 1 reportLow impact1 neutral
Applying "Parse, don't validate" pattern in Rust for type-safe invariants
Key points
- Examines "Parse, don't validate" in Rust.
- Illustrates issues with `Vec` and `Option` for guaranteed non-empty lists.
- Proposes using types to enforce invariants at compile time.
- Aims to improve code clarity and prevent runtime errors.
Introduction to "Parse, don't validate"
The "Parse, don't validate" pattern, popularized by Alexis King, advocates for using a parsing step to transform untrusted input into a type that inherently enforces desired invariants. This approach shifts validation from runtime checks to compile-time type guarantees, making illegal states unrepresentable.
This article specifically applies this pattern to the Rust programming language, seeking to find and create educational examples within the Rust ecosystem.
The Problem with Runtime Validation
The article illustrates a common problem using Rust's `Vec` type. When a function, such as `get_configuration_directories`, guarantees that a returned `Vec` is non-empty, subsequent code still needs to handle the `Option` returned by methods like `first()`. This forces redundant checks for an empty state that has already been ruled out.
This redundancy leads to less clear code, potential performance overhead from unnecessary checks, and a risk of errors if the invariant changes without updating all dependent code paths.
Enforcing Invariants with Types
The core issue is that `Vec` is a type that can fundamentally be empty. Even with a runtime check ensuring it's not empty, the type system doesn't reflect this guarantee. The "Parse, don't validate" pattern suggests creating a new type that represents a non-empty list, thereby encoding the invariant directly into the type system.
By returning a type that cannot be empty, consumers of the function would no longer need to perform checks for emptiness, as the type itself would guarantee its presence.
Benefits of Type-Driven Invariants
Implementing this pattern in Rust offers several benefits. It improves code clarity by removing redundant checks and `unreachable!` assertions. It also enhances code safety by making it impossible to represent an invalid state (e.g., an empty list when one is required) at the type level, catching errors at compile time rather than runtime.
This approach aligns with Rust's philosophy of strong type safety and helps developers write more robust and maintainable code.
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Reporting from
This article explores the "Parse, don't validate" pattern in Rust, focusing on how to enforce invariants using types rather than runtime checks. It demonstrates how Rust's type system can prevent redundant checks for conditions already guaranteed by a parsing function, improving code clarity and safety.