option-chain-tool

A Rust macro that brings JavaScript-like optional chaining to Rust, allowing you to use the ? operator in any context - even in functions that don't return Option or Result.

Motivation

In Rust, the ? operator is a powerful tool for ergonomic error handling, but it comes with a significant limitation: it only works inside functions that return Option or Result. This restriction forces developers into verbose workarounds when working with nested optional values in regular functions.

The Problem

Consider a common scenario: you have deeply nested optional structures and want to safely access a value deep within. In Rust, you typically have three options, all with drawbacks:

1. Verbose chaining with and_then and map:

let value = test_struct.value
    .and_then(|v1| v1.value)
    .and_then(|v2| v2.value);

This becomes increasingly unreadable as nesting increases.

2. Manual unwrapping with pattern matching:

if let Some(____v) = &test_struct.value {
    if let Some(____v) = &____v.value {
        Some(&____v.value)
    } else {
        None
    }
} else {
    None
}

Even more verbose and error-prone.

3. Wrapping everything in a helper function:

fn get_nested_value(test_struct: &TestStruct) -> Option<i32> {
    test_struct.value?.value?.value
}
let value = get_nested_value(&test_struct);

Creates unnecessary function overhead for simple access patterns.

The Solution

JavaScript and TypeScript developers enjoy optional chaining (?.) that makes this trivial:

const value = test_struct?.value?.value?.value;

option-chain brings this ergonomic experience to Rust with a simple macro that lets you use the natural ? operator anywhere:

let value = opt!(test_struct.value?.value?.value?);

Clean, readable, and efficient - without requiring your function to return Option.

Features

• 🪶 Lightweight: Just a use native rust TokenStream, zero dependencies.
• 🚀 Zero overhead: Compiles down to the same code as manual if let Some(...) = ... chains.
• 🎯 Intuitive: Uses Rust's familiar ? operator syntax.
• 🔒 Type-safe: Full compile-time type checking.
• 📦 Works everywhere: Use in functions returning (), concrete types, or anything else.

Installation

Add this to your Cargo.toml:

[dependencies]
option-chain-tool = "0.10"

Usage

Basic Example

use option_chain_macro::opt;

#[derive(Debug, Clone)]
struct User {
    profile: Option<Profile>,
}

#[derive(Debug, Clone)]
struct Profile {
    address: Option<Address>,
}

#[derive(Debug, Clone)]
struct Address {
    city: Option<String>,
}

fn main() {
    let user = User {
        profile: Some(Profile {
            address: Some(Address {
                city: Some("New York".to_string()),
            }),
        }),
    };

    // ✅ With opt! macro - clean and readable
    let city = opt!(user.profile?.address?.city?);
    println!("City: {:?}", city); // City: Some("New York")

    // ❌ Without macro - verbose and nested
    let city_verbose = user.profile
        .and_then(|p| p.address)
        .and_then(|a| a.city);
}

Advanced Examples

Working with Vectors and Methods

use option_chain_macro::opt;

#[derive(Debug, Clone)]
struct Team {
    members: Option<Vec<Member>>,
}

#[derive(Debug, Clone)]
struct Member {
    name: String,
    email: Option<String>,
}

fn main() {
    let team = Team {
        members: Some(vec![
            Member { 
                name: "Alice".to_string(), 
                email: Some("alice@example.com".to_string()) 
            },
            Member { 
                name: "Bob".to_string(), 
                email: None 
            },
        ]),
    };

    // Access nested vector elements safely
    let first_email = opt!(team.members?.get(0)?.email?);
    println!("First email: {:?}", first_email); // Some("alice@example.com")

    let second_email = opt!(team.members?.get(1)?.email?);
    println!("Second email: {:?}", second_email); // None
}

Accessing Required Fields Through Optional Chains

use option_chain_macro::opt;

#[derive(Debug, Clone)]
struct Config {
    database: Option<DatabaseConfig>,
}

#[derive(Debug, Clone)]
struct DatabaseConfig {
    host: String,          // Required field
    port: i32,            // Required field
    credentials: Option<Credentials>,
}

#[derive(Debug, Clone)]
struct Credentials {
    username: String,
}

fn main() {
    let config = Config {
        database: Some(DatabaseConfig {
            host: "localhost".to_string(),
            port: 5432,
            credentials: None,
        }),
    };

    // Access required fields through optional chain
    let port = opt!(&config.database?.port);
    println!("Port: {:?}", port); // Some(5432)

    let host = opt!(&config.database?.host);
    println!("Host: {:?}", host); // Some("localhost")
}

Real-World Scenario

use option_chain_macro::opt;

#[derive(Debug, Clone)]
struct ApiResponse {
    data: Option<ResponseData>,
}

#[derive(Debug, Clone)]
struct ResponseData {
    user: Option<UserData>,
}

#[derive(Debug, Clone)]
struct UserData {
    id: i32,
    metadata: Option<Metadata>,
}

#[derive(Debug, Clone)]
struct Metadata {
    last_login: Option<String>,
}

// Regular function that doesn't return Option
fn process_response(response: ApiResponse) {
    // Without opt! macro, you'd need verbose chaining or helper functions
    // With opt! macro, it's clean and expressive
    
    let user_id = opt!(&response.data?.user?.id);
    let last_login = opt!(response.data?.user?.metadata?.last_login?);
    
    println!("User ID: {:?}", user_id);           // Some(42)
    println!("Last login: {:?}", last_login);    // Some("2024-01-01")
}

Comparison

JavaScript/TypeScript:

const city = user?.profile?.address?.city;

Rust without option-chain:

let city = user.profile
    .and_then(|p| p.address)
    .and_then(|a| a.city);

Rust with option-chain:

let city = opt!(user.profile?.address?.city?);

How It Works

The opt! macro transforms your code at compile time, converting the intuitive ? syntax into efficient if let Some(...) = ... chains. There's no runtime overhead - it's purely a syntactic convenience.

// What you write:
let a: Option<&String> = opt!(user.profile?.address?.city?);

// What the compiler sees (roughly):
let a: Option<&String> = if let Some(____v) = &user.profile {
    if let Some(____v) = &____v.address {
        if let Some(____v) = &____v.city {
            Some(____v)
        } else {
            None
        }
    } else {
        None
    }
} else {
    None
};

When to Use

✅ Use option-chain when:

• Working with nested optional structures in functions that don't return Option
• You want to avoid verbose and_then chains
• Accessing data from deeply nested API responses or configs
• You need readable optional access in utility functions

❌ Consider alternatives when:

• Your function already returns Option or Result (use native ? operator)
• You're working with flat structures (no nesting)
• You need custom error messages (consider explicit error handling)

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

License

This project is licensed under the MIT License - see the LICENSE file for details.

See Also

• Rust Option documentation
• The ? operator
• Optional chaining in JavaScript