methods - 无论如何， “context”和 “with_context”有什么区别？

Question

这是有关Context的文档:

/// Wrap the error value with additional context.
fn context<C>(self, context: C) -> Result<T, Error>
where
    C: Display + Send + Sync + 'static; 

/// Wrap the error value with additional context that is evaluated lazily
/// only once an error does occur.
fn with_context<C, F>(self, f: F) -> Result<T, Error>
where
    C: Display + Send + Sync + 'static,
    F: FnOnce() -> C;

实际上，不同之处在于with_context需要关闭，如反正README所示:

use anyhow::{Context, Result};

fn main() -> Result<()> {
    // ...
    it.detach().context("Failed to detach the important thing")?;

    let content = std::fs::read(path)
        .with_context(|| format!("Failed to read instrs from {}", path))?;
    // ...
}

但是看起来我可以用with_context替换context方法，通过删除||摆脱闭包，并且程序的行为不会改变。
两种方法的内在区别是什么？

Answer 1

提供给with_context的闭包是惰性计算的，而您选择使用with_context而不是context的原因与您选择惰性计算任何事物的原因相同:它很少发生，并且计算成本很高。一旦满足这些条件，with_context就比context更可取。注释的伪示例:

fn calculate_expensive_context() -> Result<()> {
    // really expensive
    std::thread::sleep(std::time::Duration::from_secs(1));
    todo!()
}

// eagerly evaluated expensive context
// this function ALWAYS takes 1+ seconds to execute
// consistently terrible performance
fn failable_operation_eager_context(some_struct: Struct) -> Result<()> {
    some_struct
        .some_failable_action()
        .context(calculate_expensive_context())
}

// lazily evaluated expensive context
// function returns instantly, only takes 1+ seconds on failure
// great performance for average case, only terrible performance on error cases
fn failable_operation_lazy_context(some_struct: Struct) -> Result<()> {
    some_struct
        .some_failable_action()
        .with_context(|| calculate_expensive_context())
}