无论如何，“context”和“with_context”之间有什么区别？

html5 • • 问答

这是无论如何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;

在实践中，不同之处在于with_context需要一个闭包，如无论如何的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))?;
    // ...
}

但看起来我可以用 替换with_context方法context，通过删除来摆脱闭包||，并且程序的行为不会改变。

引擎盖下的两种方法有什么区别？

回答

关闭提供给 with_context是惰性求值的，您使用with_contextovercontext的原因与您选择惰性求值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，就with_context变得比 更可取context。评论伪示例：

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())
}

以上是无论如何，“context”和“with_context”之间有什么区别？的全部内容。
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