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//! # SNAFU //! //! ## Design philosophy //! //! SNAFU believes that it should be easy to bin one underlying error //! type (such as [`io::Error`](std::io::Error)) into multiple //! domain-specific errors while also optionally adding contextual //! information. //! //! SNAFU is designed to be used in libraries, not just end-user applications. //! //! ## Quick example //! //! This example mimics a (very poor) authentication process that //! opens a file, writes to a file, and checks the user's ID. While //! two of our operations involve an [`io::Error`](std::io::Error), //! these are different conceptual errors to us. //! //! SNAFU creates *context selectors* mirroring each error //! variant. These are used with the [`context`](ResultExt::context) //! method to provide ergonomic error handling. //! //! ```rust //! use snafu::{Snafu, ResultExt}; //! use std::{fs, path::{Path, PathBuf}}; //! //! #[derive(Debug, Snafu)] //! enum Error { //! #[snafu_display("Could not open config from {}: {}", "filename.display()", "source")] //! OpenConfig { filename: PathBuf, source: std::io::Error }, //! #[snafu_display("Could not save config to {}: {}", "filename.display()", "source")] //! SaveConfig { filename: PathBuf, source: std::io::Error }, //! #[snafu_display("The user id {} is invalid", "user_id")] //! UserIdInvalid { user_id: i32 }, //! } //! //! type Result<T, E = Error> = std::result::Result<T, E>; //! //! fn log_in_user<P>(config_root: P, user_id: i32) -> Result<()> //! where //! P: AsRef<Path>, //! { //! let config_root = config_root.as_ref(); //! let filename = &config_root.join("config.toml"); //! //! let config = fs::read(filename).context(OpenConfig { filename })?; //! // Perform updates to config //! fs::write(filename, config).context(SaveConfig { filename })?; //! //! if user_id != 42 { //! return Err(Error::UserIdInvalid { user_id }); //! } //! //! Ok(()) //! } //! ``` //! //! ## The `Snafu` macro //! //! This procedural macro implements the [`Error`](std::error::Error) //! trait and produces the corresponding context selectors. //! //! ### Detailed example //! //! ```rust //! use snafu::Snafu; //! use std::path::PathBuf; //! //! #[derive(Debug, Snafu)] //! enum Error { //! #[snafu_display("Could not open config at {}: {}", "filename.display()", "source")] //! OpenConfig { filename: PathBuf, source: std::io::Error }, //! #[snafu_display("Could not open config: {}", "source")] //! SaveConfig { source: std::io::Error }, //! } //! ``` //! //! #### Generated code //! //! This will generate two additional types called *context //! selectors*: //! //! ```rust,ignore //! struct OpenConfig<P> { filename: P } //! struct SaveConfig<P> { filename: P } //! ``` //! //! Notably: //! //! 1. One struct is created for each enum variant. //! 1. The name of the struct is the same as the enum variant's name. //! 1. The `source` field has been removed; the library will //! automatically handle this for you. //! 1. Each field's type has been replaced with a generic type. //! //! Each context selector will have an implementation of //! [`From`](std::convert::From) for a `snafu::Context`: //! //! ```rust,ignore //! impl<P> From<Context<Error, OpenConfig<T0>>> for Error //! where //! P: Into<PathBuf>, //! ``` pub use snafu_derive::Snafu; /// A combination of an underlying error and additional information /// about the error. It is not expected for users of this crate to /// interact with this type. pub struct Context<E, C> { /// The underlying error pub error: E, /// Information that provides a context for the underlying error pub context: C, } /// Additions to [`Result`](std::result::Result). pub trait ResultExt<T, E> { /// Extend a `Result` with additional context-sensitive information. /// /// ```rust /// use snafu::{Snafu, ResultExt}; /// /// #[derive(Debug, Snafu)] /// enum Error { /// Authenticating { user_name: String, user_id: i32, source: ApiError }, /// } /// /// fn example() -> Result<(), Error> { /// another_function().context(Authenticating { user_name: "admin", user_id: 42 })?; /// Ok(()) /// } /// /// # type ApiError = Box<dyn std::error::Error>; /// fn another_function() -> Result<i32, ApiError> { /// /* ... */ /// # Ok(42) /// } /// ``` /// /// Note that the [`From`](std::convert::From) implementation /// generated by the macro will call /// [`Into::into`](std::convert::Into::into) on each field, so the /// types are not required to exactly match. fn context<C>(self, context: C) -> Result<T, Context<E, C>>; /// Extend a `Result` with lazily-generated context-sensitive information. /// /// ```rust /// use snafu::{Snafu, ResultExt}; /// /// #[derive(Debug, Snafu)] /// enum Error { /// Authenticating { user_name: String, user_id: i32, source: ApiError }, /// } /// /// fn example() -> Result<(), Error> { /// another_function().with_context(|| Authenticating { /// user_name: "admin".to_string(), /// user_id: 42, /// })?; /// Ok(()) /// } /// /// # type ApiError = std::io::Error; /// fn another_function() -> Result<i32, ApiError> { /// /* ... */ /// # Ok(42) /// } /// ``` /// /// Note that this *may not* be needed in many cases because the /// [`From`](std::convert::From) implementation generated by the /// macro will call [`Into::into`](std::convert::Into::into) on /// each field. fn with_context<C>(self, context: impl FnOnce() -> C) -> Result<T, Context<E, C>>; } impl<T, E> ResultExt<T, E> for std::result::Result<T, E> { fn context<C>(self, context: C) -> Result<T, Context<E, C>> { self.map_err(|error| Context { error, context }) } fn with_context<C>(self, context: impl FnOnce() -> C) -> Result<T, Context<E, C>> { self.map_err(|error| { let context = context(); Context { error, context } }) } }