Bracket pattern in Rust

LICENSE

The MIT License (MIT)

Copyright (c) 2020 Richard Cook

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
the Software, and to permit persons to whom the Software is furnished to do so,
subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

main-bracket.rs

/// See https://wiki.haskell.org/Bracket_pattern
/// # Arguments
///
/// * `R` - type of resource
/// * `T` - type of return value from computation
/// * `E` - type of error
/// * `acquire` - possibly failing function that acquires resource
/// * `release` - function that releases resource
/// * `consume` - possibly failing function that consumes resource and yields return value of type `T`
#[allow(dead_code)]
pub fn bracket<R, T, E, F, G, H>(acquire: F, release: G, consume: H) -> std::result::Result<T, E>
where
    F: FnOnce() -> std::result::Result<R, E>,
    G: FnOnce(R) -> (),
    H: FnOnce(&R) -> std::result::Result<T, E>,
{
    let resource = acquire()?;
    let result = consume(&resource);
    release(resource);
    result
}

main-demo-bracket.rs

fn demo_bracket(acquire_resource: bool) -> std::io::Result<()> {
    let result: &str = bracket::<_, _, std::io::Error, _, _, _>(
        || match acquire_resource {
            true => Ok(Some(Resource::new())),
            false => Ok(None),
        },
        |resource| match resource {
            Some(r) => r.release(),
            None => {}
        },
        |_| {
            println!("Resource used");
            Err(std::io::Error::new(
                std::io::ErrorKind::Other,
                "Resource does not leak!",
            ))
        },
    )?;
    println!("result={}", result);
    Ok(())
}

main-demo-drop.rs

#[allow(unreachable_code)]
fn demo_drop(acquire_resource: bool) -> std::io::Result<()> {
    struct ResourceHolder {
        resource: Resource,
    }

    impl Drop for ResourceHolder {
        fn drop(&mut self) {
            self.resource.release()
        }
    }

    let resource = match acquire_resource {
        true => Some(Resource::new()),
        false => None,
    };
    let _holder = resource.map(|r| ResourceHolder { resource: r });

    println!("Resource used");
    return Err(std::io::Error::new(
        std::io::ErrorKind::Other,
        "Resource does not leak!",
    ));

    Ok(())
}

main-demo-explicit-drop.rs

#[allow(unreachable_code)]
#[allow(unused_variables)]
fn demo_explicit_drop(acquire_resource: bool) -> std::io::Result<()> {
    struct ResourceHolder {
        resource: Resource,
    }

    impl Drop for ResourceHolder {
        fn drop(&mut self) {
            self.resource.release()
        }
    }

    let resource = match acquire_resource {
        true => Some(Resource::new()),
        false => None,
    };
    let holder = resource.map(|r| ResourceHolder { resource: r });

    println!("Resource used");
    return Err(std::io::Error::new(
        std::io::ErrorKind::Other,
        "Resource does not leak!",
    ));

    drop(holder);

    Ok(())
}

main-demo-resource-leak.rs

#[allow(unreachable_code)]
#[allow(unused_variables)]
fn demo_resource_leak(acquire_resource: bool) -> std::io::Result<()> {
    let resource = match acquire_resource {
        true => Some(Resource::new()),
        false => None,
    };

    println!("Resource used");
    return Err(std::io::Error::new(
        std::io::ErrorKind::Other,
        "Leak the resource!",
    ));

    match resource {
        Some(r) => r.release(),
        None => {}
    };

    Ok(())
}

main-resource.rs

struct Resource {}

impl Resource {
    fn new() -> Self {
        println!("Resource acquired");
        Self {}
    }

    fn release(&self) {
        println!("Resource released");
    }
}