Lesson 19 of 20

6 mins read

Closures & Functional Iterators (map, filter, collect)

In Plain English

Closures are anonymous functions that can capture variables from their enclosing environment (|param| expr). Combined with Rust’s lazy Iterator trait adaptors (.map, .filter, .fold, .collect), they produce zero-cost functional data transformation pipelines.

The Mental Model

"Iterator adaptors are like interlocking industrial conveyor sections: .filter() drops defective parts, .map() polishes them, and .collect() packs them into a finished crate—and the whole conveyor only spins when the final crate is demanded (lazy evaluation)."

Deep Dive: How It Works

Closures: Anonymous functions defined with vertical bars: |x, y| x + y.

Lazy Iterators: Iterator adaptors do no work until a consuming method like .collect(), .sum(), or .count() is called.

.collect(): Assembles transformed iterator streams back into collections like Vec<T>.

Syntax Blueprint

let doubled: Vec<i32> = numbers.iter().filter(|&&x| x > 5).map(|x| x * 2).collect();

Chains functional transformation stages into a single optimized SIMD loop.

Core Rules to Remember

Zero-Cost Abstractions: Iterator pipelines compile to the exact same assembly as hand-tuned C loops.

Lazy Evaluation: Adaptors process elements on-demand without intermediate allocations.

Closures (|x| ...): Capture surrounding variables automatically by reference or move.

Common Beginner Traps & How to Fix Them

Mistake:Forgetting type annotation on .collect().

Why it happens: collect() can build Vec, HashSet, String, etc. The compiler needs to know target collection type.

How to fix: Use let res: Vec<i32> = ... or .collect::<Vec<_>>() (the turbofish operator).

Pro Tip: Use .into_iter() if you want to consume the collection and own the elements inside closure adaptors.

Live Interactive Example

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Composing Iterator Pipelines

Rust 1.78 / rustc

Output Console

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How it works: Demonstrates functional iterator chaining with filter, map, and collect.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Filter and double numbers with iterator

Given let numbers = vec![1, 2, 3, 4, 5];
Use .into_iter().filter(|x| x % 2 == 0).map(|x| x * 10).collect::<Vec<i32>>()
Print "Even doubled: [20, 40]".

Sandbox Output

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