How Rust spoils pattern matching

Rust is renowned as a great low-level language with a strong focus on safety. However, when exploring its pattern matching capabilities, especially with the Box type for heap allocation, certain limitations become evident.

Understanding Rust's Box Type:
In Rust, the Box type is a smart pointer used for heap allocation. It's particularly useful when you have large data that you don't want to copy or when you need to ensure a fixed size for a type that is recursive or of unknown size at compile time. Despite its usefulness, Box introduces challenges in pattern matching due to Rust’s ownership and borrowing rules.

Pattern Matching in Rust:
Pattern matching in Rust is powerful, allowing for concise and readable handling of complex data structures. But when it comes to a Box, things get tricky.

Consider this Rust example:

enum MetaOrData {
    Data(Box<[i32]>),
    Meta(String),
}

fn main() {
    let a = MetaOrData::Data(Box::new([10]));
}

MetaOrData is an enum with a variant containing a Box of an integer array. The objective is to print the first item of the array if it exists. Attempting an implementation yields:

fn print_first_data_item(it: &MetaOrData) {
    match it {
        MetaOrData::Data(Box([a])) => {
            println!("Box first item {:?}", a); 
        },
        _ => {}
    }
}

This intuitive code unfortunately won't compile due to Rust's privacy rules:

///    |         ^^^ constructor is not visible here due to private field

The error occurs because Box is a struct with a private field, preventing direct deconstruction in a pattern match.

Despite discussions in the Rust community (e.g., Rust issue #49733 and Rust issue #29641), direct pattern matching against Box hasn't been implemented, leading to workarounds:

fn print_first_data_item(it: &MetaOrData) {
    match it {
        MetaOrData::Data(data) => {
            let slice = &**data;  // Dereferencing the box to access its content
            if let [a] = slice {
                println!("Box first item {:?}", a);
            }
        },
        _ => {}
    }
}

Here, we dereference the Box to access its content. While effective, this approach can be less intuitive and slightly cumbersome.

How it should work or OCaml's Approach to Pattern Matching: A Comparison:

Actual code

OCaml’s pattern matching is straightforward, allowing direct deconstruction of lists in match arms, enhancing code readability and simplicity.

Conclusion:
While Rust offers robust safety features and powerful pattern matching capabilities, its handling of Box types in pattern matches can be restrictive and verbose. Conversely, OCaml provides a more straightforward pattern matching syntax. Integrating Rust features in OCaml with modes, as explored in Jane Street's blog, shows the potential for combining these languages' strengths.