Curriculum Data modeling Enums as tagged unions exercise 3 · mcq-explain

Enums as tagged unions

A function accepts a "kind" parameter from a small fixed set: pending, ready, failed. Pick the STRONGEST reason to model it as a Rust enum rather than a String.

TypeScript reference

Enums are faster to compare because no string comparison is involved — minor perf difference; not the structural reason.

The enum makes the legal set EXPLICIT to the compiler; match is checked for exhaustiveness, so a typo or a new state is a compile error rather than a silent fallthrough at runtime.

Enums use less memory because variants are stored as integers — true but minor; correctness, not size, is the win.

Rust does not have a String type at the function-parameter level; you must use an enum — false; &str and String work fine as parameters.

About this theme

Rust enums are real sum types: each variant can carry different data. This is the closest Rust analogue to TypeScript discriminated unions, but the compiler enforces exhaustiveness. Planned exercises: 1. choose enum syntax. 2. recognise tuple-like and struct-like variants. 3. choose an enum over string tags. 4. fill a variant constructor. 5. fill an enum type annotation. 6. write a small enum. 7. write a value of a data-carrying variant. 8. translate a TS discriminated union. 9. model a small state machine with enum variants.