How to specify type in field assigning with "?"?

help
1

I have a function fn natOrNone<U, E, F: FnOnce(i32) -> Result<U, E>>(n: i32, f: F) -> Result<Option<U>, E> and I am using it in SomeStruct { theField: natOrNone(0, |x| Ok(x))? } . Clearly, rustc could not know what type E is. But where should I put the type signature ::<Result<Option<i32>, MyError>? I tried some places like I normally do with function calls. All got grammar error.

2

This struct is created inside some function, and the ? operator is doing the early return from this function. So, this error type (or something it can be implicitly coerced to) must be in its signature.

3

Yes, the signature is there. The whole function (creating the struct) is:

impl FromStr for SomeStruct {
  type Err = MyError;
  fn from_str(s: &str) -> Result<Self, Self::Err> {
    Ok(SomeStruct { theField: natOrNone(0, |x| Ok(x))? })
  }
}

And the error is:
(under natOrNone) cannot infer type for E.

4 
    Ok(SomeStruct { theField: natOrNone::<_, SomeE, _>(0, |x| Ok(x))? })
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