Option<T> in Rust: 15 Examples from Beginner to Advanced

A Code-First Guide with Runnable Examples

🚧 This post is under construction 🚧

This is Episode 02

The Posts Of The Saga


Table of Contents

🔴 - Example 10 - Borrowing Instead of Moving - .as_ref() and .as_mut()

Real-world context

Inspecting Option<T> without consuming it, modifying in-place, reusing Option<T> after checking.

Runnable Example

Copy and paste in Rust Playground

// cargo run --example 10ex

fn main() {
    // i32 implements Copy, so Option<i32> also implements Copy
    let opt = Some(42);

    // Pattern matching copies opt instead of moving it
    if let Some(n) = opt {
        println!("{n}"); // n is copied from the Option
    }
    println!("{:?}", opt); // OK: opt was copied, not moved

    println!();
    // String does NOT implement Copy, so Option<String> does not implement Copy either
    let opt = Some(String::from("hello"));

    // Pattern matching moves opt and its inner String
    if let Some(s) = opt {
        // s is moved out of opt
        println!("Length: {}", s.len());
    }
    // println!("{:?}", opt); // ERROR: opt was moved and cannot be used here

    // Borrowing with as_ref => Option<T> remains usable afterwards
    println!();
    let opt = Some(String::from("hello"));
    if let Some(s) = opt.as_ref() {
        println!("Length: {}", s.len());
    }
    println!("{:?}", opt); // Ah, ha, ha, ha, stayin' alive, stayin' alive

    println!();
    // this express the same intention as `as_ref`
    if let Some(my_str) = &opt {
        println!("Length: {}", my_str.len());
    }
    println!("{:?}", opt); // Ah, ha, ha, ha, stayin' alive, stayin' alive

    println!();
    let mut path = Some(std::env::current_dir().expect("Cannot read current dir"));

    // as_ref() is useful with map - read without consuming
    let len = path.as_ref().map(|p| p.as_os_str().len());
    println!("The path {:?} has a length of {:?}", path, len);

    // as_mut is useful with map - modify in place
    path.as_mut().map(|p| p.push("documents"));
    path.as_mut().map(|p| p.push("top_secret"));

    println!("The path is now: {:?}", path);
}

Read it Aloud

“as_ref() converts Option<T> to Option<&T>, so that we can peek inside without consuming. as_mut() gives Option<&mut T> for peek and poke. Both leave the original Option<T> intact.”

Comments

IMPORTANT The Option<T> is considered moved unless it implements Copy trait, which only happens if T implements Copy. Review Example 02 now that you have that in mind.

  • With the first if let Some(n) = opt, the Option<T> on the right-hand side is copied and opt remains available.
    • i32 implements the Copy trait
    • so Option<i32> also implements Copy
    • so using opt in a pattern match copies the Option<T> instead of moving it
    • opt remains valid.
  • With the second if let Some(s) = opt, the Option<T> on the right-hand side is moved, which means opt is no longer available afterward.
    • Option<String> does not implement the Copy trait
    • so opt is moved into the if let Some(s)
    • we cannot use opt afterward

This explains the need for tools to “read” and to “read/write” inside Option<T>

  • Option<T>.as_ref() provides an immutable reference to the value inside the Option<T>
  • Option<T>.as_mut() provides a mutable reference to the value inside the Option<T>.
  • In both cases, the Option<T> itself is not consumed, but any changes made through the mutable reference do modify the underlying value.
  • An alternative to if let Some(s) = opt.as_ref() {... is if let Some(s) = &opt {...
  • The line path.as_mut().map(|p| p.push("documents")); generates a warning. Do you know why? How can you simplify the code?

Key Points

  1. Signature: as_ref(&self) -> Option<&T>, as_mut(&mut self) -> Option<&mut T>
  2. When to use: Reading Option multiple times, modifying without replacing
  3. With map: opt.as_ref().map(|val| ...) lets us transform without moving
  4. Ownership: Original Option keeps ownership - crucial for reuse

Find More Examples

Regular expressions to use either in VSCode ou Powershell: \.as_ref\(\)\.map or \.as_mut\(\)

🔴 - Example 11 - Extracting Value and Leaving None - .take()

Real-world context

Consuming resources (files, connections), state machines, cleanup operations, RAII.

Runnable Example

Copy and paste in Rust Playground

use std::fs::{self, File};

struct Editor {
    file: Option<File>,
}

impl Editor {
    fn is_open(&self) -> bool {
        self.file.is_some()
    }

    fn close(&mut self) {
        if let Some(_f) = self.file.take() {
            // _f is File, self.file is now None automatically
            println!("Closing file");
            // _f is dropped and the file is automatically closed at the end of the block
        }
    }
}

fn main() {
    let mut editor = Editor {
        file: Some(File::create("temp.txt").expect("Failed to create temp.txt")),
    };
    println!("Is open: {}", editor.is_open()); // true
    editor.close();
    println!("Is open: {}", editor.is_open()); // false

    // Clean up: remove temp.txt if it exists
    if fs::metadata("temp.txt").is_ok() {
        fs::remove_file("temp.txt").expect("Failed to delete temp.txt");
        println!("temp.txt deleted");
    }
}

Copy and paste in Rust Playground. This example demonstrates how to implement RAII (resource acquisition is initialization) with the help of take() even if early release are allowed.

struct Resource {
    name: String,
}

impl Resource {
    fn new(name: &str) -> Self {
        println!("\t[{}] Acquired", name);
        Self {
            name: name.to_string(),
        }
    }
}

impl Drop for Resource {
    fn drop(&mut self) {
        println!("\t[{}] Released", self.name);
    }
}

struct Guard {
    resource: Option<Resource>,
}

impl Guard {
    fn new(name: &str) -> Self {
        Self {
            resource: Some(Resource::new(name)),
        }
    }

    // Manually release before scope ends
    fn release(&mut self) {
        if let Some(r) = self.resource.take() {
            println!("\t[{}] Early release", r.name);
        } // r is dropped here
    }
}

impl Drop for Guard {
    fn drop(&mut self) {
        if self.resource.is_some() {
            println!("\tGuard dropped with resource still held");
        }
        // resource.take() not needed here - Option<T> drops automatically
    }
}

fn main() {
    println!("Example 1: Auto release at scope end");
    {
        let _guard = Guard::new("DB Connection");
        println!("\tDoing some work...");
    } // _guard dropped here, Resource released

    println!("\nExample 2: Early release with take()");
    {
        let mut guard = Guard::new("File Handle");
        println!("\tDoing work...");
        guard.release(); // Release early via take()
        println!("\tMore work after release...");
    } // guard dropped, but resource already gone
}

Read it Aloud

Option<T>.take() says: “Give me the value inside Some(v) + replace the Option<T> with None + and return the value as Option<T>.”

Comments

  • Option<T>.take() is a move + automatic None assignment in one operation.
  • First example
    • The code at the end of main() is here to make sure the file is deleted if it exists.
  • RAII
    • The Option<Resource> + take() pattern is idiomatic for managing resources that we may want to release manually while maintaining RAII safety.

Key Points

  1. Signature: take(&mut self) -> Option<T> - requires mutable reference
  2. Atomic: Extracts value and sets to None in one step (prevents use-after-move bugs)
  3. Common use: Cleanup, state transitions, resource management
  4. vs moving: if let Some(x) = opt.take() vs if let Some(x) = opt (latter moves entire Option)

Find More Examples

VSCode search: \.take\(\) (regex) Regular expression to use either in VSCode ou Powershell: \.take\(\)

🔴 - Example 12 - Conditional Mapping - .filter()

Real-world context

Validation, keeping only values that meet criteria, sanitization.

Runnable Example

Copy and paste in Rust Playground

fn main() {
    let numbers = vec![Some(1), Some(15), Some(25), None, Some(5)];

    // Filter keeps only Some(v) where the predicate is true
    let filtered: Vec<Option<i32>> = numbers.iter().map(|opt| opt.filter(|&n| n > 10)).collect();
    println!("Raw numbers: {:?}", numbers);  // [Some(1), Some(15), Some(25), None, Some(5)]
    println!("Filtered   : {:?}", filtered); // [None,    Some(15), Some(25), None, None]

    // Combining with map
    let name = Some("  Zoubida  ");
    let result = name
        .map(|n| n.trim())
        .filter(|n| !n.is_empty())  // Keep only if not empty after trim
        .map(|n| n.to_uppercase());
    println!("{:?}", result); // Some("ZOUBIDA")

    // Filter out invalid values
    let maybe_age = Some(-5);
    let valid_age = maybe_age.filter(|&age| age >= 0 && age <= 150);
    println!("{:?}", valid_age); // None (negative age rejected)
}

Read it Aloud

filter(|val| condition) says: “If Option<T> is Some(v) and the condition is true, keep it as Some(v). Otherwise, return None. It’s like map() but it can remove values.”

Comments

  • At this point it is important to read the story that the signatures tell us. In VSCode if we hover over the word .filter we can read :
core::option::Option
impl<T> Option<T>
pub const fn filter<P>(self, predicate: P) -> Self
where
    P: FnOnce(&T) -> bool + Destruct,
    T: Destruct,

I know what you think but this is like riding under the rain on track. No one like that but… This is an investment with large ROI, especially the next week-end in Belgium where it rains 370 days/year. Ok… Above we learn that, on an Option<T>, the predicate P acts on &T (did you notice the P: FnOnce(&T)?). The predicate borrows the tested values it does’nt consume them.

And what about .map()? Let’s play the game and we read:

core::iter::traits::iterator::Iterator
pub trait Iterator
pub fn map<B, F>(self, f: F) -> Map<Self, F>
where
    Self: Sized,
    F: FnMut(Self::Item) -> B,

Now, just to make sure we know what we are talking about while talking about the first filter… Copy and paste in Rust Playground the code below:

// cargo run --example 12_2ex

fn main() {
    let numbers = vec![Some(1), Some(15), Some(25), None, Some(5)];

    // Filter keeps only Some(v) where the predicate is true
    let filtered: Vec<Option<i32>> = numbers.iter().map(|&opt| opt.filter(|&n| n > 10)).collect();
    println!("Raw numbers: {:?}", numbers); // [Some(1), Some(15), Some(25), None, Some(5)]
    println!("Filtered   : {:?}", filtered); // [None,   Some(15), Some(25), None, None]

    // Filter keeps only Some(v) where the predicate is true
    let filtered: Vec<Option<i32>> = numbers.iter().map(|opt| opt.filter(|&n| n > 10)).collect();
    println!("Raw numbers: {:?}", numbers); // [Some(1), Some(15), Some(25), None, Some(5)]
    println!("Filtered   : {:?}", filtered); // [None,   Some(15), Some(25), None, None]

    // Filter keeps only Some(v) where the predicate is true
    let filtered: Vec<Option<i32>> = numbers.iter().map(|opt| opt.filter(|n| *n > 10)).collect();
    println!("Raw numbers: {:?}", numbers); // [Some(1), Some(15), Some(25), None, Some(5)]
    println!("Filtered   : {:?}", filtered); // [None,   Some(15), Some(25), None, None]
}

If you use VSCode, feel free to press CTRL+ALT to reveal the types and you should see:


All three versions above compile and produce identical results, but they differ in how they handle references in the closure parameters. Let me break down each one:

They all start with numbers.iter(). Reading the help of .iter() we learn the following:

core::slice
impl<T> [T]
pub const fn iter(&self) -> Iter<'_, T>
T = Option<i32>
  1. Note that rust-analyzer is smart enough to recall us that in our case, T = Option<i32>.
  2. We should also note that even if we call .iter() on a vector, the documentation explains that it will be considered as a slice ([T]).
  3. The return type is Iter<'_, T>. The first element is a lifetime specifier. It indicates that the iterator is “bound” to the lifetime of the object on which we call iter() (here, numbers, the vector/slide of Option<T>). Indeed, slice iterators borrow elements instead of moving them.

So at this point we know we will get a core::slice::Iter<'_, T> but what is precisely the returned type? We have to go on the Rust documentation and look for the struct core::slice::Iter


Once on the page of the Struct Iter, in the section Trait Implementations we look, in alphabetical order, for the implementation of the Iterator trait:


This is where we learn that the implementation define an associated type Item: type Item = &'a T. This means that the iterator of a slice of elements of type T produces references (&) to these elements, with the same lifetime ('a) as the iterator itself. To make a long story short, the iterator produces &Option<i32>

Now let’s analyze the 3 different versions of the line:

Version 1: |&opt| opt.filter(|&n| n > 10)

  • Here we destructure the reference in the closure parameter. Since numbers.iter() yields &Option<i32>, using |&opt| gives us opt: Option<i32> (a copy, since i32 is Copy). Then |&n| destructures the &i32 from filter’s closure to get n: i32.

Version 2: |opt| opt.filter(|&n| n > 10)

  • Here opt is &Option<i32>, but thanks to Rust’s auto-deref, calling .filter() works seamlessly. The |&n| pattern still destructures the reference inside.

Version 3: |opt| opt.filter(|n| *n > 10) * Here we keep the reference and explicitly dereference with *n when comparing.

Which one to prefer?

  • We should keep the first part: numbers.iter().map(|opt| ...)
  • For integers or other Copy types:
    • |&n| n > 10 is concise and idiomatic.
  • For general types (non-Copy):
    • Use |n| *n > 10 to avoid forcing a copy or clone.

V2 is better but type specific. Personally I would vote for V3 since it can be generalized.

Key Points

  1. Signature: filter<P>(self, predicate: P) -> Option<T> where P: FnOnce(&T) -> bool
  2. Chainable: Combine with map for “transform then validate”
  3. None handling: None stays None (predicate never called)
  4. vs if: More functional, composable with other Option<T> methods

Find More Examples

Regular expression to use either in VSCode ou Powershell: \.filter\(\s*\|[^|]+\|[^)]*\)

🔴 - Example 13 - Working with Collections of Options - .flatten() and .filter_map()

Real-world context

Processing results where some operations fail, removing None values, transforming + filtering.

Runnable Example

Copy and paste in Rust Playground

fn parse_number(s: &str) -> Option<i32> {
    s.parse().ok()
}

fn main() {
    let inputs = vec!["42", "invalid", "100", "", "7"];

    // Method 1: map + flatten
    let numbers: Vec<i32> = inputs
        .iter()
        .map(|&s| parse_number(s))  // Vec<Option<i32>>
        .flatten()                  // Remove None, unwrap Some
        .collect();

    println!("{:?}", numbers);      // [42, 100, 7]

    // Method 2: filter_map (more efficient)
    let numbers2: Vec<i32> = inputs
        .iter()
        .filter_map(|&s| parse_number(s))
        .collect();

    println!("{:?}", numbers2);     // [42, 100, 7]

    // With transformation
    let doubled: Vec<i32> = inputs
        .iter()
        .filter_map(|&s| parse_number(s).map(|n| n * 2))
        .collect();

    println!("{:?}", doubled);      // [84, 200, 14]
}

Read it Aloud

”.flatten() converts Vec<Option<T>> to Vec<T> by discarding None while .filter_map(|x| optional_transform(x)) combines .map() and .flatten() in one step.”

Comments

  • .filter_map(|x| optional_transform(x)) is more efficient for large collections

Key Points

  1. flatten: Iterator<Item = Option<T>> → Iterator<Item = T>
  2. filter_map: Combines filter + map - one pass instead of two
  3. Performance: filter_map avoids intermediate allocation
  4. Common pattern: Processing lists where operations might fail

Find More Examples

Regular expression to use either in VSCode ou Powershell: \.flatten\(\) or \.filter_map\(

🔴 - Example 14 - Combining Multiple Options

Real-world context

Validation requiring multiple fields, coordinate systems, multi-factor authentication.

Runnable Example

Copy and paste in Rust Playground

// Method 1: Using ? operator
fn add_options(a: Option<i32>, b: Option<i32>) -> Option<i32> {
    Some(a? + b?)  // If either is None, return None immediately
}

// Method 2: Explicit match
fn add_options_match(a: Option<i32>, b: Option<i32>) -> Option<i32> {
    match (a, b) {
        (Some(x), Some(y)) => Some(x + y),
        _ => None,  // If either is None
    }
}

// Method 3: Chaining
fn add_options_and_then(a: Option<i32>, b: Option<i32>) -> Option<i32> {
    a.and_then(|x| b.map(|y| x + y))
}

fn main() {
    println!("{:?}", add_options(Some(5), Some(10)));  // Some(15)
    println!("{:?}", add_options(Some(5), None));      // None
    println!("{:?}", add_options(None, Some(10)));     // None

    // All three methods are equivalent
    assert_eq!(add_options(Some(2), Some(3)), Some(5));
    assert_eq!(add_options_match(Some(2), Some(3)), Some(5));
    assert_eq!(add_options_and_then(Some(2), Some(3)), Some(5));

    // Real-world: combining coordinates
    fn distance(x: Option<f64>, y: Option<f64>) -> Option<f64> {
        Some((x? * x? + y? * y?).sqrt())
    }

    println!("{:?}", distance(Some(3.0), Some(4.0))); // Some(5.0)
    println!("{:?}", distance(Some(3.0), None));      // None
}

Read it Aloud

“Some(a? + b?) offers a concise early-return logic to Options<T> 2 or more option. If all Options<T> are Some(v) the processing takes place, otherwise, if any is None, early reply None.”

Comments

Key Points

  1. ? operator method: Cleanest for 2+ Options - reads left to right
  2. match method: Most explicit - good for complex conditions
  3. and_then method: Functional style - harder to read for multiple values
  4. All-or-nothing: Result is Some(v) only if ALL inputs are Some(v)

Find More Examples

Regular expression to use either in VSCode ou Powershell: Some\(.+?\?\s*[+\-*/]\s*.+?\?\)

🔴 - Example 15 - Converting Option<&T> to Option<T> - .copied() and .cloned()

Real-world context

Working with references from collections, avoiding lifetime issues, simplifying ownership.

Runnable Example

Copy and paste in Rust Playground

fn main() {
    let vec = vec![1, 2, 3, 4, 5];

    // vec.first() returns Option<&i32>
    let first_ref: Option<&i32> = vec.first();
    println!("{:?}", first_ref); // Some(&1)

    // Need Option<i32> not Option<&i32>
    let first_owned: Option<i32> = vec.first().copied();
    println!("{:?}", first_owned); // Some(1) - no reference

    // With String (not Copy, requires cloned)
    let strings = vec!["hello".to_string(), "world".to_string()];
    let first_string: Option<String> = strings.first().cloned();
    println!("{:?}", first_string); // Some("hello")

    // Practical: avoiding lifetime errors
    fn get_first_double(numbers: &Vec<i32>) -> Option<i32> {
        numbers.first().copied().map(|n| n * 2)
        // Without copied(): would return Option<i32> borrowing from numbers
        // With copied(): returns owned i32, no lifetime issues
    }

    let nums = vec![10, 20, 30];
    println!("{:?}", get_first_double(&nums)); // Some(20)
}

Read it Aloud

”.copied() duplicates the value inside Option<&T> to produce Option<T> (requires the Copy trait). .cloned() does the same but uses the Clone trait instead - works for non-Copy types like String.”

Comments

Key Points

  1. When to use: Converting Option<&T> from collections to owned Option<T>
  2. copied(): For Copy types (i32, f64, char, etc.) - cheap bitwise copy
  3. cloned(): For Clone types (String, Vec, etc.) - potentially expensive
  4. Lifetime escape: Lets us return Option<T> without lifetime parameters

Find More Examples

Regular expression to use either in VSCode ou Powershell: \.copied\(\), \.cloned\(\) or \.first\(\)\.copied\(\).

Some Pitfalls and How to Avoid Them

Pitfall 1: unwrap() vs expect() vs unwrap_or() 

let opt: Option<i32> = None;

// NOK unwrap() - panics on None with generic message
// let val = opt.unwrap(); // panics: "called `Option::unwrap()` on a `None` value"

// WARN expect() - panics with custom message (better for debugging)
// let val = opt.expect("Expected a value here!"); // panics: "Expected a value here!"

// OK unwrap_or() - provides fallback, never panics
let val = opt.unwrap_or(42);
println!("{}", val); // 42

We should never use unwrap() in production. Use expect() only when None is truly impossible (with good message). Prefer unwrap_or() or proper matching.

Pitfall 2: copied() vs cloned() Confusion 

let numbers = vec![1, 2, 3];

// OK copied() for Copy types (i32)
let first: Option<i32> = numbers.first().copied();

let strings = vec!["a".to_string()];

// NOK copied() doesn't work on String (not Copy)
// let s: Option<String> = strings.first().copied(); // ERROR

// OK cloned() for Clone types
let s: Option<String> = strings.first().cloned();

We should use copied() for primitive types, cloned() for heap-allocated types (String, Vec, etc.).

Pitfall 3: Moving vs Borrowing 

let opt = Some(String::from("hello"));

// NOK This moves opt
// match opt {
//     Some(s) => println!("{}", s),
//     None => {}
// }
// println!("{:?}", opt); // ERROR: opt was moved

// OK Borrow with as_ref()
match opt.as_ref() {
    Some(s) => println!("{}", s),
    None => {}
}
println!("{:?}", opt); // Works!

We should use as_ref() when we need to inspect Option<T> without consuming it.

Pitfall 4: Understanding Some(x?) 

fn parse_and_wrap(s: &str) -> Option<Option<i32>> {
    // NOK Confusing nested Option
    Some(s.parse().ok())
}

fn parse_correctly(s: &str) -> Option<i32> {
    // OK Flatten with ?
    Some(s.parse().ok()?)
}

fn main() {
    println!("{:?}", parse_and_wrap("42"));     // Some(Some(42)) - awkward
    println!("{:?}", parse_correctly("42"));    // Some(42) - clean
    println!("{:?}", parse_correctly("invalid")); // None
}

Some(x?) means “try to get x, if None short-circuit. Otherwise wrap in Some”. Avoids nested Options.

Quick Reference & Cheat Sheet

Extraction Methods

Method Returns on Some(v) Returns on None Panics? Example
unwrap() T - ✅ Yes 01
expect(msg) T - ✅ Yes (with msg) 11
unwrap_or(default) T default ❌ No 05
unwrap_or_else(f) T f() ❌ No (lazy) 05
unwrap_or_default() T T::default() ❌ No 05

Transformation Methods

Method Type Transform Lazy? Use When Example
map(f) Option<T> → Option<U> Yes Transform always succeeds 07
and_then(f) Option<T> → Option<U> Yes Transform returns Option 08
filter(p) Option<T> → Option<T> Yes Conditional keeping 12
flatten() Option<Option<T>> → Option<T> Yes Remove nesting 13

Borrowing Methods

Method Converts Mutates Original? Example
as_ref() Option<T> → Option<&T> ❌ No 10
as_mut() Option<T> → Option<&mut T> ✅ Yes (value inside) 10
take() Option<T> → Option<T> ✅ Yes (sets to None) 11

Checking Methods

Method Returns Use When Example
is_some() bool Only need to know if Some(v) 01
is_none() bool Only need to know if None NA
is_some_and(f) bool Check Some(v) + condition NA

Note About the Performances

  • Lazy evaluation: unwrap_or_else, map, and_then, filter - closures only run when needed
  • Eager evaluation: unwrap_or - argument always evaluated
  • Zero-cost: as_ref(), as_mut(), is_some(), is_none() - compile to no-ops or simple checks

Webliography

Official Documentation

The Posts Of The Saga

Back to top

Published on: Dec 5 2025 at 10:00 AM | Last updated: Dec 5 2025 at 10:00 AM

Copyright © 1964-2025 - 40tude