结果失败返回`map_err`

Question

我正在弄清楚为什么返回时以下失败map_err：

Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
  --> src/main.rs:37:5
   |
36 | fn R() -> Result<i32, MyError> {
   |           -------------------- expected `std::result::Result<i32, MyError>` because of return type
37 |     decode().map_err(|e| Err(MyError {v: "changed".to_string() }))
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `MyError`, found enum `std::result::Result`
   |
   = note: expected enum `std::result::Result<_, MyError>`
              found enum `std::result::Result<_, std::result::Result<_, MyError>>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground`.

如果我match手动使用并映射它，它就可以工作。

代码：

Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
  --> src/main.rs:37:5
   |
36 | fn R() -> Result<i32, MyError> {
   |           -------------------- expected `std::result::Result<i32, MyError>` because of return type
37 |     decode().map_err(|e| Err(MyError {v: "changed".to_string() }))
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `MyError`, found enum `std::result::Result`
   |
   = note: expected enum `std::result::Result<_, MyError>`
              found enum `std::result::Result<_, std::result::Result<_, MyError>>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground`.

Answer 1

如果您查看签名，map_err您会发现它需要一个接受错误类型并返回错误类型的闭包，而不是Result.

decode().map_err(|e| MyError {v: "changed".to_string() })