rust - How do I conditionally check if an enum is one variant or another?

Question

I have an enum with two variants:

enum DatabaseType {
    Memory,
    RocksDB,
}

What do I need in order to make a conditional if inside a function that checks if an argument is DatabaseType::Memory or DatabaseType::RocksDB?

fn initialize(datastore: DatabaseType) -> Result<V, E> {
    if /* Memory */ {
        //..........
    } else if /* RocksDB */ {
        //..........
    }
}

Answer 1

First, go back and re-read the free, official Rust book: The Rust Programming Language, specifically the chapter on enums.

---

match

fn initialize(datastore: DatabaseType) {
    match datastore {
        DatabaseType::Memory => {
            // ...
        }
        DatabaseType::RocksDB => {
            // ...
        }
    }
}

if let

fn initialize(datastore: DatabaseType) {
    if let DatabaseType::Memory = datastore {
        // ...
    } else {
        // ...
    }
}

==

#[derive(PartialEq)]
enum DatabaseType {
    Memory,
    RocksDB,
}

fn initialize(datastore: DatabaseType) {
    if DatabaseType::Memory == datastore {
        // ...
    } else {
        // ...
    }
}

matches!

This is available since Rust 1.42.0

fn initialize(datastore: DatabaseType) {
    if matches!(datastore, DatabaseType::Memory) {
        // ...
    } else {
        // ...
    }
}

See also:

• How to compare enum without pattern matching
• Read from an enum without pattern matching
• Compare enums only by variant, not value
• https://doc.rust-lang.org/std/macro.matches.html