Curriculum Collections & iteration Closures exercise 2 · mcq-explain

Closures

The closure |x| x * 2 declares no type for x. Pick the explanation that names why it compiles.

TypeScript reference

Closures default to i32; the parameter is implicitly typed i32 unless annotated otherwise.

The compiler defaults closure parameters to &i32; an explicit type is needed only for non-numeric inputs.

The compiler INFERS the parameter type from the closure's first concrete use. Once inferred, the closure is monomorphic — calling it later with a different type is a compile error.

Rust closures are dynamically typed; x accepts any type at runtime — there is no static check.

About this theme

Rust closures look like compact TypeScript arrow functions, but they capture by borrow, mutable borrow, or move depending on use. Planned exercises: 1. choose closure syntax. 2. recognise inferred parameter types. 3. choose move only when ownership must transfer. 4. fill a predicate closure. 5. fill a mapping closure. 6. write a closure used with sort_by_key. 7. write a closure that captures a threshold. 8. translate TS arrow functions in array methods. 9. fix a closure capture that conflicts with a later borrow.