我正在尝试编写一个必须打开文件的库,我想处理Result
使用的std::fs::File::create
类型。在这个包装函数中,我无法弄清楚如何匹配返回结果:
use std::fs::File;
use std::path::Path;
use std::fs::File;
use std::path::Path;
pub fn allocate(path:& str) -> File{
let mut file = File::create(Path::new(path));
}
然后调用:
mod whisper;
use std::io::Write;
fn main(){
let mut handle = whisper::allocate("./a_file.wsp");
match handle {
Ok(_) => println!("success!"),
Err(e) => println!("sorry, got {}",e),
}
return;
}
但由于类型不匹配,代码无法编译:
Xaviers-MacBook-Pro:graphite-rust xavierlange$ cargo build
Compiling graphite-rust v0.0.1 (file:///Users/xavierlange/code/viasat/graphite-rust)
src/main.rs:8:5: 8:10 error: mismatched types:
expected `std::fs::File`,
found `core::result::Result<_, _>`
(expected struct `std::fs::File`,
found enum `core::result::Result`) [E0308]
src/main.rs:8 Ok(_) => println!("hi!"),
^~~~~
src/main.rs:9:5: 9:11 error: mismatched types:
expected `std::fs::File`,
found `core::result::Result<_, _>`
(expected struct `std::fs::File`,
found enum `core::result::Result`) [E0308]
src/main.rs:9 Err(e) => println!("sorry, got {}",e),
^~~~~~
error: aborting due to 2 previous errors
Could not compile `graphite-rust`.
std::fs::File::create
的签名是fn create<P: AsRef<Path>>(path: P) -> Result<File>
,所以我不应期待&#34;展开&#34;使用匹配的值?为什么期望File
值?
答案 0 :(得分:7)
让我们看一下代码的简化版本MCVE。在编程时创建小例子非常有用,因为它可以帮助您一次专注于一个问题:
use std::fs::File;
use std::path::Path;
fn allocate(path: &str) -> File {
File::create(Path::new(path))
}
fn main() {}
(我也冒昧地将你的代码与普遍的Rust风格对齐;我强烈建议你学习它并喜欢它以便更好地进行社区互动!)
时会出现同样的错误<anon>:5:5: 5:34 error: mismatched types:
expected `std::fs::File`,
found `core::result::Result<std::fs::File, std::io::error::Error>`
(expected struct `std::fs::File`,
found enum `core::result::Result`) [E0308]
<anon>:5 File::create(Path::new(path))
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
问题是因为您已将函数的返回类型定义为File
,但函数体返回Result
!
fn allocate(path: &str) -> File
您需要确保函数上的类型以及函数的对齐方式。最简单的是unwrap
结果,这会导致线程在失败情况下出现恐慌。
fn allocate(path: &str) -> File {
File::create(Path::new(path)).unwrap()
}
您还可以返回自己的Result
,然后强制调用者处理失败(我的首选):
use std::io;
fn allocate(path: &str) -> io::Result<File> {
File::create(Path::new(path))
}
查看错误的另一种方法是这一半:
use std::fs::File;
fn allocate() -> File { unimplemented!() }
fn main() {
match allocate() {
Ok(..) => println!("OK!"),
Err(..) => println!("Bad!"),
}
}
在这里,我们尝试在match
上File
,但File
不是包含变体Ok
和Err
的枚举 - 这将是Result
!因此,您会收到一个错误,表明:
<anon>:7:9: 7:15 error: mismatched types:
expected `std::fs::File`,
found `core::result::Result<_, _>`
(expected struct `std::fs::File`,
found enum `core::result::Result`) [E0308]
<anon>:7 Ok(..) => println!("OK!"),
^~~~~~