fehler 1.0.0-alpha.1

A small, opinionated error-handling library

Der Fehler

Der Fehler is a small but very opinionated Rust error handling library.

In many ways, der Fehler is a successor to failure. However, unlike failure, der Fehler is built around the standard library's Error trait, which has adopted the necessary improvements that the Fail trait had provided thanks to RFC 2504.

Der Fehler provocatively and unapologetically uses the terminology of exceptions.

Der Fehler provides these items:

The Exception type

Exception is a polymorphic error type, essentially a trait object. It is similar to the failure::Error type, but for the Error trait in std. There are a few key improvements:

  • Like failure::Error is possible to construct an ad hoc error from any type that implements Debug and Display (such as a string). Unlike failure::Error, it can be properly downcast to that type.
  • Unlike failure::Error, Exception is guaranteed to be the size of a single narrow pointer (failure::Error is the size of a wide pointer).

Otherwise, it is roughly the same type: an Error trait object that guarantees the presence of a backtrace.

The throw! macro

throw! is a macro which is equivalent to the Err($e)? pattern. It takes an error type and "throws" it.

The #[throws] attribute

The throws attribute modifies a function or method to make it return a Result. It takes an optional typename as an argument to the attribute which will be the error type of this function; if no typename is supplied, the error type is Exception.

Within the function body, returns (including the implicit final return) are automatically "Ok-wrapped." To raise errors, use ? or the throws! macro.

For example, these two functions are equivalent:

fn foo(x: bool) -> i32 {
    if x {
    } else {

fn bar(x: bool) -> Result<i32, i32> {
    if x {
    } else {

The error! macro

This macro constructs an ad hoc error from format strings, similar to the format! macro.

The Context trait

This crate also defines a Context trait for the Result type, which contains a context method for injecting context around an error.


  • Possibly add a Display derive
  • Make throws work on closures and async blocks (attributes are not allowed on expressions on stable)


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.