GoKawiilBlog post applies 'Parse, don't validate' idiom to Rust code patterns
A developer's blog post examines the 'Parse, don't validate' programming pattern, originally described by Alexis King for Haskell, and translates it into Rust examples. The post uses a case study involving a configuration-directory reader that validates a vector is non-empty, then shows how callers must still handle an 'unreachable' None case despite the invariant already being checked.
GoKawiil's interpretation of the reporting above, not reported fact.
The post argues that validating data without transforming its type leaves later code forced to re-handle cases that should be logically impossible, which can introduce unreachable!() calls or redundant checks. This suggests that encoding invariants directly into Rust's type system, such as returning a custom non-empty vector type, could let the compiler enforce guarantees rather than relying on runtime checks and programmer discipline.
- The post adapts a Haskell-focused idiom, 'Parse, don't validate,' to Rust programming practices.
- A worked example shows a function that validates a vector is non-empty but still requires callers to handle a theoretically impossible empty case.
- The discussion points toward using Rust's type system to encode invariants so illegal states become unrepresentable rather than just checked.
Source: eli.thegreenplace.net, 2026-09-27
Published there as: “Rusty thoughts on "Parse, don't validate"”
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