derive(Toss)

Tiny helper library for thiserror that generates traits to conveniently handle errors.

[dependencies]
tosserror = "0.1"

Compiler support: requires rustc 1.56+

Table of Contents

• Exaple Usage
  • Comparison with conventional map_err
• How it works
• Why use derive(Toss)
  • Brevity
  • Convenience with autocompletion
  • Why you may not use derive(Toss)
• Attributes
  • thiserror's attributes: #[source], #[from], #[backtrace]
  • #[visibility]
    • Examples
    • Tip: how to use error cross-module/project-wide
  • #[prefix]
• Features
  • thiserror
• Generated Code from derive(Toss)
• Credits

Example Usage

use thiserror::Error;
use tosserror::Toss;

#[derive(Error, Toss, Debug)]
pub enum DataStoreError {
    #[error("invalid value ({value}) encountered")]
    InvalidValue {
      value: i32,
      source: std::num::TryFromIntError,
    },
    #[error("data store disconnected with msg {msg}: {status}")]
    Disconnect{
      status: u8,
      msg: String,
      source: std::io::Error
    }
}

// uses
get_value().toss_invalid_value(123)?;

// lazily provide context
data_store_fn().toss_disconnect_with(|| (123, "some msg".to_owned()))?;

Comparison with conventional map_err

get_value().map_err(|e| DataStoreError::InvalidValue {
  value: 123,
  source: e
})?;

data_store_fn().map_err(|e| DataStoreError::Disconnect {
    status: 123,
    msg: "some msg".to_owned(),
    source: e
})?;

How it works

derive(Toss) works by creating a trait for every enum variant.

For the error below:

#[derive(Error, Toss, Debug)]
pub enum DataStoreError {
    #[error("invalid value ({value}) encountered")]
    InvalidValue {
      value: i32,
      source: std::num::TryFromIntError,
    },
    #[error("data store disconnected with msg {msg}: {status}")]
    Disconnect {
      status: u8,
      msg: String,
      source: std::io::Error
    }
}

it will generate traits and implementations as below:

// pseudo generated code
trait TossDataStoreErrorInvalidValue<T> {
    fn toss_invalid_value(self, value: i32) -> Result<T, DataStoreError>;
}
impl<T> TossDataStoreErrorInvalidValue<T> for Result<T, std::num::TryFromIntError> {
    fn toss_invalid_value(self, value: i32) -> Result<T, DataStoreError> { ... }
}

trait TossDataStoreErrorDisconnect<T> {
    fn toss_disconnect(self, status: u8, msg: String) -> Result<T, DataStoreError>;
}
impl<T> TossDataStoreErrorDisconnect<T> for Result<T, std::io::Error> {
    fn toss_disconnect(self, status: u8, msg: String) -> Result<T, DataStoreError> { ... }
}

With these traits generated, you get auto-completion of .toss_invalid_value(i32) to any method that returns Result<T, TryFromError>. Same applies to .toss_disconnect(u8, String) for Result<T, io::Error>.

If you want these traits be visible to other modules, see #[visibility].

For the full generated code example, see Generated Code from derive(Toss).

Why use derive(Toss)

Brevity

When I've used libraries like thiserror, I've noticed that handling the errors add significant bloat to the code, and hinders readability.

You may encounter simple function calls followed by 2 to 4 lines of error handling like below:

simple_function().map_err(|e| LongError::InvalidValue {
    context1: 123u32,
    context2: "some context".to_owned(),
    source: e
})?;

When the error handling is longer than the procedure itself, your code's primary focus is no longer the logic; it's handling errors.

With derive(Toss), your error handling code will almost always stay in one line:

simple_function().toss_invalid_value(123u32, "some context".to_owned())?;

You can also pass a closure that returns the arguments to lazily evaluate context values.

simple_function().toss_invalid_value_with(|| (123u32, "some context".to_owned()))?;

Convenience with autocompletion

With thiserror, it may be cumbersome to type out all the characters:

1. .map_err
2. two |s
3. enum name
4. variant name
5. field names and values

I won't say these are the major hurdles of programming, but it does get pretty annoying when you have to type these over and over.

With derive(Toss), you get auto-completion!

When you type .toss_, you will get list of suggestions that only apply to the underlying error type.

For example, if your method returns io::Error, the suggestion will show only the handler methods that take in io::Error.

Why you may not use derive(Toss)

With the upsides, I admit there is a downside that may make you not want to use this library.

it will obscure the code

In my reddit post asking for feedback on this idea, pretty common feedback was this.

This macro creates many traits that you cannot see how it's defined or implemented. If you're a new contributor to some open source library, and you encountered uses of these methods here and there, and you had no idea about tosserror, you could be very confused what it means, what it does and how it works.

Compared to this, .map_err and .with_context provides a clear source that you can easily see what it does and how it works.

This is a fair criticism, and, if this is a big concern, you will not want to use this library.

To many people, using .map_err is just not a big enough inconvenience to justify obscuring your codebase.

For those of you who still want to give it a try, here are my thoughts on this concern:

1. if you use the rust-analyzer lsp, it will pretty clearly outline the definition, arguments, and return type,
2. the library is simple enough that, newcomers to the library will pretty easily figure out what it does, or, you can just let them know that this thing exists.

Attributes

thiserror's attributes: #[source], #[from], #[backtrace]

The library uses the same rules as thiserror to determine the source and backtrace fields.

If you define a field as source or backtrace, either by field name or attribute, it will be excluded from the generated method's arguments.

#[derive(Error, Toss, Debug)]
#[error("my error with value {value}")]
pub struct MyError {
  value: i32,
  source: io::Error,
  backtrace: std::backtrace::Backtrace
}

// pseudo generated code
trait TossMyError<T> {
    fn toss_invalid_value(self, value: i32) -> Result<T, MyError>;
}
impl<T> TossMyError<T> for io::Error {
    fn toss_invalid_value(self, value: i32) -> Result<T, MyError> {
      ...
    }
}

#[visibility]

By default, generated traits are private, only visible to the module it's created in.

With #[visibility], you can expose the generated traits to other modules or to public.

You can either place the attribute above the enum to apply to all the traits generated for the error, or place it above specific variants to apply it to specific variants' generated traits.

Examples

#[derive(Error, Toss, Debug)]
#[error("...")]
#[visibility(pub)]
pub enum Error1 {
  Var1 { ... }, // generates trait `pub trait TossError1Var1`
  Var2 { ... }  // generates trait `pub trait TossError1Var2`
}

#[derive(Error, Toss, Debug)]
#[error("...")]
#[visibility(pub(super))]
pub enum Error2 {
  Var1 { ... }, // generates trait `pub(super) trait TossError2Var1`
  #[visibility(pub(crate))]
  Var2 { ... }  // generates trait `pub(crate) trait TossError2Var2`
}

Tip: how to use error cross-module/project-wide

Many libraries tend to define one error in the root module, and use it throughout the modules in the library.

This may be troublesome as auto-completion of traits may not work in different modules. This means that you would have to manually import these traits, when you don't even know what they look like, or import all (use *) from that module.

In this case, here's what I usually do:

1. create a new module error and put the error definition in it.

   pub(crate) mod error {
     #[derive(Error, Toss, Debug)]
     #[visibility(pub(crate))]
     pub enum MyError {
       ...
     }
   }
   

   Make sure the specify the visibility attribute to pub(crate) or whatever necessary.

2. whenever you use error handling, just import the error module

   use crate::error::*;
   

   this way, you can cleanly import all generated traits along with your error.

#[prefix]

There may be cases where there are multiple errors in the module, and the variant names clash.

In this case, compiler will complain about the ambiguous method name.

With #[prefix], you can prefix a text value to the generated trait methods.

Simply place #[prefix] to prefix the snake_cased enum name as the prefix value, or place #[prefix(custom_prefix)] to write your own prefix value.

#[derive(Error, Toss, Debug)]
#[error("...")]
#[prefix] // apply prefix "connect" (enum name without `_error`) to all variants
pub enum ConnectError {
  Var1 { ... }, // generates trait method `fn toss_connect_var1(self)`
  Var2 { ... }  // generates trait method `fn toss_connect_var2(self)`
}

#[derive(Error, Toss, Debug)]
#[error("...")]
#[prefix(custom)] // apply prefix "custom" to all variants
pub enum AnotherError {
  Var1 { ... }, // generates trait method `fn toss_custom_var1(self)`
  #[prefix(specific)] // apply prefix "specific" just to this variant
  Var2 { ... }  // generates trait method `fn toss_specific_var2(self)`
}

Features

thiserror

[dependencies]
tosserror = { version = "0.1", features = ["thiserror"] }
# thiserror = "1.0" # no longer necessary

Enabling feature thiserror re-exports thiserror::Error so that you don't have to depend both on tosserror and thiserror.

You can now just use tosserror::Error to derive your errors.

use tosserror::{Error, Toss};

#[derive(Error, Toss, Debug)]
pub enum DataStoreError {
    #[error("invalid value ({value}) encountered")]
    InvalidValue {
      value: i32,
      source: std::num::TryFromIntError,
    },
}

Should this be a default feature? Let me know by leaving a thumbs up to set "thiserror" as a default feature PR.

limitations

This works by re-exporting thiserror within the generated code of derive(Toss).

Therefore, derive(tosserror::Error) only works when used together with derive(Toss).

Generated Code from derive(Toss)

Example error

use thiserror::Error;
use tosserror::Toss;

#[derive(Error, Toss, Debug)]
pub enum DataStoreError {
    #[error("invalid value ({value}) encountered")]
    InvalidValue {
      value: i32,
      source: std::num::TryFromIntError,
    },
    #[error("data store disconnected with msg {msg}: {status}")]
    #[visibility(pub(crate))]
    Disconnect{
      status: u8,
      msg: String,
      source: std::io::Error
    }
}

Generated code

trait TossDataStoreErrorInvalidValue<__RETURN> {
    fn toss_invalid_value(self, value: i32) -> Result<__RETURN, DataStoreError>;
    fn toss_invalid_value_with<F: FnOnce() -> (i32)>(
        self,
        f: F,
    ) -> Result<__RETURN, DataStoreError>;
}
impl<__RETURN> TossDataStoreErrorInvalidValue<__RETURN>
for Result<__RETURN, std::num::TryFromIntError> {
    fn toss_invalid_value(self, value: i32) -> Result<__RETURN, DataStoreError> {
        self.map_err(|e| {
            DataStoreError::InvalidValue {
                source: e,
                value,
            }
        })
    }
    fn toss_invalid_value_with<F: FnOnce() -> (i32)>(
        self,
        f: F,
    ) -> Result<__RETURN, DataStoreError> {
        self.map_err(|e| {
            let (value) = f();
            DataStoreError::InvalidValue {
                source: e,
                value,
            }
        })
    }
}

pub(crate) trait TossDataStoreErrorDisconnect<__RETURN> {
    fn toss_disconnect(
        self,
        status: u8,
        msg: String,
    ) -> Result<__RETURN, DataStoreError>;
    fn toss_disconnect_with<F: FnOnce() -> (u8, String)>(
        self,
        f: F,
    ) -> Result<__RETURN, DataStoreError>;
}
impl<__RETURN> TossDataStoreErrorDisconnect<__RETURN>
for Result<__RETURN, std::io::Error> {
    fn toss_disconnect(
        self,
        status: u8,
        msg: String,
    ) -> Result<__RETURN, DataStoreError> {
        self.map_err(|e| {
            DataStoreError::Disconnect {
                source: e,
                status,
                msg,
            }
        })
    }
    fn toss_disconnect_with<F: FnOnce() -> (u8, String)>(
        self,
        f: F,
    ) -> Result<__RETURN, DataStoreError> {
        self.map_err(|e| {
            let (status, msg) = f();
            DataStoreError::Disconnect {
                source: e,
                status,
                msg,
            }
        })
    }
}

Credits

This library takes a lot of implementation details and code structures from thiserror, as it is a complementary library for thiserror and works on the same attributes.

Special thanks to the creator and maintainers of thiserror for creating such a simple, clean, easy-to-read yet great library.