Error Handling

Propagate and transform failures with Result, Option, and the ? operator. Results appear in the same fence: same-line // comments when short, multiline // blocks below the sample when not.

Question Mark Operator

? returns early on Err / None (in Option context).

fn parse(s: &str) -> Result<i32, std::num::ParseIntError> {
    Ok(s.parse()?)
}
parse("12").unwrap()  // 12

map_err Context

Transform error types while keeping Ok.

fn go(s: &str) -> Result<i32, String> {
    s.parse::<i32>().map_err(|e| e.to_string())
}
go("x").unwrap_err().contains("invalid")  // true

Option ok_or

Lift Option into Result with an error value.

None::<i32>.ok_or("missing").unwrap_err()  // "missing"
Some(1).ok_or("missing").unwrap()  // 1

unwrap and expect

Panic on failure - fine in examples, rare in libraries.

Some(3).unwrap()  // 3
Some(3).expect("need value")  // 3

unwrap_or Default

Provide a fallback without panicking.

None::<i32>.unwrap_or(0)  // 0
"x".parse::<i32>().unwrap_or(-1)  // -1

and_then Chain

Sequence fallible steps.

Some(2).and_then(|n| Some(n * 3))  // Some(6)

map Ok Value

Transform success without touching Err.

Ok::<_, ()>(2).map(|n| n + 1)  // Ok(3)

Custom Error Enum

Domain errors as enums with Display/Error.

#[derive(Debug)]
enum E { NotFound, Bad }
impl std::fmt::Display for E {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{self:?}")
    }
}
format!("{}", E::NotFound)  // "NotFound"

From Conversion

? uses From to convert error types.

fn inner() -> Result<(), String> { Err("e".into()) }
fn outer() -> Result<(), String> { Ok(inner()?) }
outer().unwrap_err()  // "e"

collect Results

Collect iterator of Results into one Result.

let xs: Result<Vec<_>, _> = ["1", "2"].iter().map(|s| s.parse::<i32>()).collect();
xs.unwrap()  // [1, 2]

anyhow Context Note

In apps, crates like anyhow add context chains - pattern: attach context on ?.

// anyhow::Context::context(result, "loading config")
// keeps error cause chain for operators

thiserror Derive Note

Library errors often use thiserror for Display/From derives.

// #[derive(thiserror::Error, Debug)]
// enum E { #[error("not found")] NotFound }

panic Unwind

panic! aborts the current thread path; prefer Result in APIs.

// panic!("bug");  // unwinds or aborts depending on profile

Result Alias

Type aliases shorten repeated Result signatures.

type R<T> = Result<T, String>;
fn f() -> R<i32> { Ok(1) }
f().unwrap()  // 1

Inspect Err

Debug side effects without consuming Result (nightly/inspect_err or match).

let r: Result<i32, &str> = Err("x");
match &r {
    Err(e) => *e,  // "x"
    Ok(_) => "",
}

Pattern Matching - match on Result
Traits and Generics - From conversions
Collections and Iterators - collect Result

The Rust Programming Language (The Book) - Canonical beginner-to-intermediate guide (ownership, borrowing, traits, etc.).
Compiler Error Index - Explanations of every rustc error code.
Standard Library (std) Docs - Complete API reference for the standard library.
The Rust Programming Language, 3rd Edition (No Starch) - No Starch Press.
Fearless Rust (Leanpub) - Freek van Keulen.

Stack versions: Rust 1.97.0 · edition 2024 · Tokio 1.x · serde 1.0

Revisado por Chris St. John·Última actualización: 19 jul 2026

Was this article helpful?

Reference Code Reference Result Error Rust

Error Handling

Propagate and transform failures with Result, Option, and the ? operator. Results appear in the same fence: same-line // comments when short, multiline // blocks below the sample when not.

Question Mark Operator

? returns early on Err / None (in Option context).

fn parse(s: &str) -> Result<i32, std::num::ParseIntError> {
    Ok(s.parse()?)
}
parse("12").unwrap()  // 12

map_err Context

Transform error types while keeping Ok.

fn go(s: &str) -> Result<i32, String> {
    s.parse::<i32>().map_err(|e| e.to_string())
}
go("x").unwrap_err().contains("invalid")  // true

Option ok_or

Lift Option into Result with an error value.

None::<i32>.ok_or("missing").unwrap_err()  // "missing"
Some(1).ok_or("missing").unwrap()  // 1

unwrap and expect

Panic on failure - fine in examples, rare in libraries.

Some(3).unwrap()  // 3
Some(3).expect("need value")  // 3

unwrap_or Default

Provide a fallback without panicking.

None::<i32>.unwrap_or(0)  // 0
"x".parse::<i32>().unwrap_or(-1)  // -1

and_then Chain

Sequence fallible steps.

Some(2).and_then(|n| Some(n * 3))  // Some(6)

map Ok Value

Transform success without touching Err.

Ok::<_, ()>(2).map(|n| n + 1)  // Ok(3)

Custom Error Enum

Domain errors as enums with Display/Error.

#[derive(Debug)]
enum E { NotFound, Bad }
impl std::fmt::Display for E {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{self:?}")
    }
}
format!("{}", E::NotFound)  // "NotFound"

From Conversion

? uses From to convert error types.

fn inner() -> Result<(), String> { Err("e".into()) }
fn outer() -> Result<(), String> { Ok(inner()?) }
outer().unwrap_err()  // "e"

collect Results

Collect iterator of Results into one Result.

let xs: Result<Vec<_>, _> = ["1", "2"].iter().map(|s| s.parse::<i32>()).collect();
xs.unwrap()  // [1, 2]

anyhow Context Note

In apps, crates like anyhow add context chains - pattern: attach context on ?.

// anyhow::Context::context(result, "loading config")
// keeps error cause chain for operators

thiserror Derive Note

Library errors often use thiserror for Display/From derives.

// #[derive(thiserror::Error, Debug)]
// enum E { #[error("not found")] NotFound }

panic Unwind

panic! aborts the current thread path; prefer Result in APIs.

// panic!("bug");  // unwinds or aborts depending on profile

Result Alias

Type aliases shorten repeated Result signatures.

type R<T> = Result<T, String>;
fn f() -> R<i32> { Ok(1) }
f().unwrap()  // 1

Inspect Err

Debug side effects without consuming Result (nightly/inspect_err or match).

let r: Result<i32, &str> = Err("x");
match &r {
    Err(e) => *e,  // "x"
    Ok(_) => "",
}

Pattern Matching - match on Result
Traits and Generics - From conversions
Collections and Iterators - collect Result

The Rust Programming Language (The Book) - Canonical beginner-to-intermediate guide (ownership, borrowing, traits, etc.).
Compiler Error Index - Explanations of every rustc error code.
Standard Library (std) Docs - Complete API reference for the standard library.
The Rust Programming Language, 3rd Edition (No Starch) - No Starch Press.
Fearless Rust (Leanpub) - Freek van Keulen.

Stack versions: Rust 1.97.0 · edition 2024 · Tokio 1.x · serde 1.0

Revisado por Chris St. John·Última actualización: 19 jul 2026

Was this article helpful?

Error Handling

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Error Handling