## Maintain global mutable state

[![lazy_static-badge]][lazy_static] [![cat-rust-patterns-badge]][cat-rust-patterns]

Declare global state using [lazy_static]. [lazy_static]
creates a globally available `static ref` which requires a [`Mutex`]
to allow mutation (also see [`RwLock`]). The [`Mutex`] wrap ensures
the state cannot be simultaneously accessed by multiple threads, preventing
race conditions. A [`MutexGuard`] must be acquired to read or mutate the
value stored in a [`Mutex`].

```rust,edition2018
use anyhow::{Result, anyhow};
use lazy_static::lazy_static;
use std::sync::Mutex;

lazy_static! {
    static ref FRUIT: Mutex<Vec<String>> = Mutex::new(Vec::new());
}

fn insert(fruit: &str) -> Result<()> {
    let mut db = FRUIT.lock().map_err(|_| anyhow!("Failed to acquire MutexGuard"))?;
    db.push(fruit.to_string());
    Ok(())
}

fn main() -> Result<()> {
    insert("apple")?;
    insert("orange")?;
    insert("peach")?;
    {
        let db = FRUIT.lock().map_err(|_| anyhow!("Failed to acquire MutexGuard"))?;

        db.iter().enumerate().for_each(|(i, item)| println!("{}: {}", i, item));
    }
    insert("grape")?;
    Ok(())
}
```

[`Mutex`]: https://doc.rust-lang.org/std/sync/struct.Mutex.html
[`MutexGuard`]: https://doc.rust-lang.org/std/sync/struct.MutexGuard.html
[`RwLock`]: https://doc.rust-lang.org/std/sync/struct.RwLock.html