Crate nullable_result

A replacement for Option<Result<T, E>> or Result<Option<T>, E>.

Sometimes, no value returned from an operation is not an error. It’s a special case that needs to be handled, but it’s separate from error handling. Wrapping an Option in a Result or vice versa can get very confusing very fast. Instead, use a NullableResult.

This is how it’s defined:

pub enum NullableResult<T, E> {
    Ok(T),
    Err(E),
    None,
}

Convert to and From std Types

It defines the From trait for Option<Result<T, E>> and for Result<Option<T>, E> in both directions, so you can easily convert between the standard library types and back.

use nullable_result::NullableResult;

let opt_res: Option<Result<usize, isize>> = Some(Ok(4));
let nr: NullableResult<usize, isize> = NullableResult::from(opt_res);
let opt_res: Option<Result<_,_>> = nr.into();

Normally, you don’t need to annotate the types so much, but in this example, there’s not enough info for the compiler to infer the types.

Unwrapping Conversions

There are also methods for unwrapping conversions. (i.e. convert to Option<T> or Result<T, E>. When converting a a Result, you need to provide an error value in case the NullableResult contains None.

use nullable_result::NullableResult;
let nr = NullableResult::<usize, isize>::Ok(5);
let opt: Option<usize> = nr.clone().option();
let res: Result<usize, isize> = nr.result(-5);

Extract the Value

The crate comes with a convenience macro extract that works like the ? operator and cna be used in functions that return a NullableResult as long as the error type is the same. It takes a NullableResult, if it contains an Ok value, the value is extracted and returned, if it contains an Err or None, the function returns early with the Err or None wrapped in a new NullableResult.

use nullable_result::{NullableResult, extract};

fn do_a_thing() -> NullableResult<usize, isize> {
    let res = some_other_func();
    let number = extract!(res); // <---- this will cause the function to return early
    NullableResult::Ok(number as usize + 5)
}

// note that the two functions have different types for their Ok values
fn some_other_func() -> NullableResult<i8, isize> {
    NullableResult::None
}

Macros

extract

Structs

TryFilter

TryFilterMap

Enums

NullableResult

Traits

GeneralIterExt

IterExt