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move Formatting Traits down

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Ralf Jung 2019-10-12 13:31:58 +02:00
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commit f727f8ae5e
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src/liballoc/fmt.rs
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@ -97,209 +97,6 @@
//! actual object being formatted, and the number of characters must have the //! actual object being formatted, and the number of characters must have the
//! type [`usize`]. //! type [`usize`].
//! //!
//! ## Formatting traits
//!
//! When requesting that an argument be formatted with a particular type, you
//! are actually requesting that an argument ascribes to a particular trait.
//! This allows multiple actual types to be formatted via `{:x}` (like [`i8`] as
//! well as [`isize`]). The current mapping of types to traits is:
//!
//! * *nothing* ⇒ [`Display`]
//! * `?` ⇒ [`Debug`]
//! * `x?` ⇒ [`Debug`] with lower-case hexadecimal integers
//! * `X?` ⇒ [`Debug`] with upper-case hexadecimal integers
//! * `o` ⇒ [`Octal`](trait.Octal.html)
//! * `x` ⇒ [`LowerHex`](trait.LowerHex.html)
//! * `X` ⇒ [`UpperHex`](trait.UpperHex.html)
//! * `p` ⇒ [`Pointer`](trait.Pointer.html)
//! * `b` ⇒ [`Binary`]
//! * `e` ⇒ [`LowerExp`](trait.LowerExp.html)
//! * `E` ⇒ [`UpperExp`](trait.UpperExp.html)
//!
//! What this means is that any type of argument which implements the
//! [`fmt::Binary`][`Binary`] trait can then be formatted with `{:b}`. Implementations
//! are provided for these traits for a number of primitive types by the
//! standard library as well. If no format is specified (as in `{}` or `{:6}`),
//! then the format trait used is the [`Display`] trait.
//!
//! When implementing a format trait for your own type, you will have to
//! implement a method of the signature:
//!
//! ```
//! # #![allow(dead_code)]
//! # use std::fmt;
//! # struct Foo; // our custom type
//! # impl fmt::Display for Foo {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! # write!(f, "testing, testing")
//! # } }
//! ```
//!
//! Your type will be passed as `self` by-reference, and then the function
//! should emit output into the `f.buf` stream. It is up to each format trait
//! implementation to correctly adhere to the requested formatting parameters.
//! The values of these parameters will be listed in the fields of the
//! [`Formatter`] struct. In order to help with this, the [`Formatter`] struct also
//! provides some helper methods.
//!
//! Additionally, the return value of this function is [`fmt::Result`] which is a
//! type alias of [`Result`]`<(), `[`std::fmt::Error`]`>`. Formatting implementations
//! should ensure that they propagate errors from the [`Formatter`][`Formatter`] (e.g., when
//! calling [`write!`]). However, they should never return errors spuriously. That
//! is, a formatting implementation must and may only return an error if the
//! passed-in [`Formatter`] returns an error. This is because, contrary to what
//! the function signature might suggest, string formatting is an infallible
//! operation. This function only returns a result because writing to the
//! underlying stream might fail and it must provide a way to propagate the fact
//! that an error has occurred back up the stack.
//!
//! An example of implementing the formatting traits would look
//! like:
//!
//! ```
//! use std::fmt;
//!
//! #[derive(Debug)]
//! struct Vector2D {
//! x: isize,
//! y: isize,
//! }
//!
//! impl fmt::Display for Vector2D {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! // The `f` value implements the `Write` trait, which is what the
//! // write! macro is expecting. Note that this formatting ignores the
//! // various flags provided to format strings.
//! write!(f, "({}, {})", self.x, self.y)
//! }
//! }
//!
//! // Different traits allow different forms of output of a type. The meaning
//! // of this format is to print the magnitude of a vector.
//! impl fmt::Binary for Vector2D {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! let magnitude = (self.x * self.x + self.y * self.y) as f64;
//! let magnitude = magnitude.sqrt();
//!
//! // Respect the formatting flags by using the helper method
//! // `pad_integral` on the Formatter object. See the method
//! // documentation for details, and the function `pad` can be used
//! // to pad strings.
//! let decimals = f.precision().unwrap_or(3);
//! let string = format!("{:.*}", decimals, magnitude);
//! f.pad_integral(true, "", &string)
//! }
//! }
//!
//! fn main() {
//! let myvector = Vector2D { x: 3, y: 4 };
//!
//! println!("{}", myvector); // => "(3, 4)"
//! println!("{:?}", myvector); // => "Vector2D {x: 3, y:4}"
//! println!("{:10.3b}", myvector); // => " 5.000"
//! }
//! ```
//!
//! ### `fmt::Display` vs `fmt::Debug`
//!
//! These two formatting traits have distinct purposes:
//!
//! - [`fmt::Display`][`Display`] implementations assert that the type can be faithfully
//! represented as a UTF-8 string at all times. It is **not** expected that
//! all types implement the [`Display`] trait.
//! - [`fmt::Debug`][`Debug`] implementations should be implemented for **all** public types.
//! Output will typically represent the internal state as faithfully as possible.
//! The purpose of the [`Debug`] trait is to facilitate debugging Rust code. In
//! most cases, using `#[derive(Debug)]` is sufficient and recommended.
//!
//! Some examples of the output from both traits:
//!
//! ```
//! assert_eq!(format!("{} {:?}", 3, 4), "3 4");
//! assert_eq!(format!("{} {:?}", 'a', 'b'), "a 'b'");
//! assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\"");
//! ```
//!
//! ## Related macros
//!
//! There are a number of related macros in the [`format!`] family. The ones that
//! are currently implemented are:
//!
//! ```ignore (only-for-syntax-highlight)
//! format! // described above
//! write! // first argument is a &mut io::Write, the destination
//! writeln! // same as write but appends a newline
//! print! // the format string is printed to the standard output
//! println! // same as print but appends a newline
//! eprint! // the format string is printed to the standard error
//! eprintln! // same as eprint but appends a newline
//! format_args! // described below.
//! ```
//!
//! ### `write!`
//!
//! This and [`writeln!`] are two macros which are used to emit the format string
//! to a specified stream. This is used to prevent intermediate allocations of
//! format strings and instead directly write the output. Under the hood, this
//! function is actually invoking the [`write_fmt`] function defined on the
//! [`std::io::Write`] trait. Example usage is:
//!
//! ```
//! # #![allow(unused_must_use)]
//! use std::io::Write;
//! let mut w = Vec::new();
//! write!(&mut w, "Hello {}!", "world");
//! ```
//!
//! ### `print!`
//!
//! This and [`println!`] emit their output to stdout. Similarly to the [`write!`]
//! macro, the goal of these macros is to avoid intermediate allocations when
//! printing output. Example usage is:
//!
//! ```
//! print!("Hello {}!", "world");
//! println!("I have a newline {}", "character at the end");
//! ```
//! ### `eprint!`
//!
//! The [`eprint!`] and [`eprintln!`] macros are identical to
//! [`print!`] and [`println!`], respectively, except they emit their
//! output to stderr.
//!
//! ### `format_args!`
//!
//! This is a curious macro which is used to safely pass around
//! an opaque object describing the format string. This object
//! does not require any heap allocations to create, and it only
//! references information on the stack. Under the hood, all of
//! the related macros are implemented in terms of this. First
//! off, some example usage is:
//!
//! ```
//! # #![allow(unused_must_use)]
//! use std::fmt;
//! use std::io::{self, Write};
//!
//! let mut some_writer = io::stdout();
//! write!(&mut some_writer, "{}", format_args!("print with a {}", "macro"));
//!
//! fn my_fmt_fn(args: fmt::Arguments) {
//! write!(&mut io::stdout(), "{}", args);
//! }
//! my_fmt_fn(format_args!(", or a {} too", "function"));
//! ```
//!
//! The result of the [`format_args!`] macro is a value of type [`fmt::Arguments`].
//! This structure can then be passed to the [`write`] and [`format`] functions
//! inside this module in order to process the format string.
//! The goal of this macro is to even further prevent intermediate allocations
//! when dealing formatting strings.
//!
//! For example, a logging library could use the standard formatting syntax, but
//! it would internally pass around this structure until it has been determined
//! where output should go to.
//!
//! # Formatting Parameters //! # Formatting Parameters
//! //!
//! Each argument being formatted can be transformed by a number of formatting //! Each argument being formatted can be transformed by a number of formatting
@ -457,7 +254,7 @@
//! //!
//! # Syntax //! # Syntax
//! //!
//! Below, you can find the full grammar of format strings. //! To summarize, you can find the full grammar of format strings.
//! The syntax for the formatting language used is drawn from other languages, //! The syntax for the formatting language used is drawn from other languages,
//! so it should not be too alien. Arguments are formatted with Python-like //! so it should not be too alien. Arguments are formatted with Python-like
//! syntax, meaning that arguments are surrounded by `{}` instead of the C-like //! syntax, meaning that arguments are surrounded by `{}` instead of the C-like
@ -480,6 +277,209 @@
//! parameter := argument '$' //! parameter := argument '$'
//! ``` //! ```
//! //!
//! # Formatting traits
//!
//! When requesting that an argument be formatted with a particular type, you
//! are actually requesting that an argument ascribes to a particular trait.
//! This allows multiple actual types to be formatted via `{:x}` (like [`i8`] as
//! well as [`isize`]). The current mapping of types to traits is:
//!
//! * *nothing* ⇒ [`Display`]
//! * `?` ⇒ [`Debug`]
//! * `x?` ⇒ [`Debug`] with lower-case hexadecimal integers
//! * `X?` ⇒ [`Debug`] with upper-case hexadecimal integers
//! * `o` ⇒ [`Octal`](trait.Octal.html)
//! * `x` ⇒ [`LowerHex`](trait.LowerHex.html)
//! * `X` ⇒ [`UpperHex`](trait.UpperHex.html)
//! * `p` ⇒ [`Pointer`](trait.Pointer.html)
//! * `b` ⇒ [`Binary`]
//! * `e` ⇒ [`LowerExp`](trait.LowerExp.html)
//! * `E` ⇒ [`UpperExp`](trait.UpperExp.html)
//!
//! What this means is that any type of argument which implements the
//! [`fmt::Binary`][`Binary`] trait can then be formatted with `{:b}`. Implementations
//! are provided for these traits for a number of primitive types by the
//! standard library as well. If no format is specified (as in `{}` or `{:6}`),
//! then the format trait used is the [`Display`] trait.
//!
//! When implementing a format trait for your own type, you will have to
//! implement a method of the signature:
//!
//! ```
//! # #![allow(dead_code)]
//! # use std::fmt;
//! # struct Foo; // our custom type
//! # impl fmt::Display for Foo {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! # write!(f, "testing, testing")
//! # } }
//! ```
//!
//! Your type will be passed as `self` by-reference, and then the function
//! should emit output into the `f.buf` stream. It is up to each format trait
//! implementation to correctly adhere to the requested formatting parameters.
//! The values of these parameters will be listed in the fields of the
//! [`Formatter`] struct. In order to help with this, the [`Formatter`] struct also
//! provides some helper methods.
//!
//! Additionally, the return value of this function is [`fmt::Result`] which is a
//! type alias of [`Result`]`<(), `[`std::fmt::Error`]`>`. Formatting implementations
//! should ensure that they propagate errors from the [`Formatter`][`Formatter`] (e.g., when
//! calling [`write!`]). However, they should never return errors spuriously. That
//! is, a formatting implementation must and may only return an error if the
//! passed-in [`Formatter`] returns an error. This is because, contrary to what
//! the function signature might suggest, string formatting is an infallible
//! operation. This function only returns a result because writing to the
//! underlying stream might fail and it must provide a way to propagate the fact
//! that an error has occurred back up the stack.
//!
//! An example of implementing the formatting traits would look
//! like:
//!
//! ```
//! use std::fmt;
//!
//! #[derive(Debug)]
//! struct Vector2D {
//! x: isize,
//! y: isize,
//! }
//!
//! impl fmt::Display for Vector2D {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! // The `f` value implements the `Write` trait, which is what the
//! // write! macro is expecting. Note that this formatting ignores the
//! // various flags provided to format strings.
//! write!(f, "({}, {})", self.x, self.y)
//! }
//! }
//!
//! // Different traits allow different forms of output of a type. The meaning
//! // of this format is to print the magnitude of a vector.
//! impl fmt::Binary for Vector2D {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! let magnitude = (self.x * self.x + self.y * self.y) as f64;
//! let magnitude = magnitude.sqrt();
//!
//! // Respect the formatting flags by using the helper method
//! // `pad_integral` on the Formatter object. See the method
//! // documentation for details, and the function `pad` can be used
//! // to pad strings.
//! let decimals = f.precision().unwrap_or(3);
//! let string = format!("{:.*}", decimals, magnitude);
//! f.pad_integral(true, "", &string)
//! }
//! }
//!
//! fn main() {
//! let myvector = Vector2D { x: 3, y: 4 };
//!
//! println!("{}", myvector); // => "(3, 4)"
//! println!("{:?}", myvector); // => "Vector2D {x: 3, y:4}"
//! println!("{:10.3b}", myvector); // => " 5.000"
//! }
//! ```
//!
//! ### `fmt::Display` vs `fmt::Debug`
//!
//! These two formatting traits have distinct purposes:
//!
//! - [`fmt::Display`][`Display`] implementations assert that the type can be faithfully
//! represented as a UTF-8 string at all times. It is **not** expected that
//! all types implement the [`Display`] trait.
//! - [`fmt::Debug`][`Debug`] implementations should be implemented for **all** public types.
//! Output will typically represent the internal state as faithfully as possible.
//! The purpose of the [`Debug`] trait is to facilitate debugging Rust code. In
//! most cases, using `#[derive(Debug)]` is sufficient and recommended.
//!
//! Some examples of the output from both traits:
//!
//! ```
//! assert_eq!(format!("{} {:?}", 3, 4), "3 4");
//! assert_eq!(format!("{} {:?}", 'a', 'b'), "a 'b'");
//! assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\"");
//! ```
//!
//! # Related macros
//!
//! There are a number of related macros in the [`format!`] family. The ones that
//! are currently implemented are:
//!
//! ```ignore (only-for-syntax-highlight)
//! format! // described above
//! write! // first argument is a &mut io::Write, the destination
//! writeln! // same as write but appends a newline
//! print! // the format string is printed to the standard output
//! println! // same as print but appends a newline
//! eprint! // the format string is printed to the standard error
//! eprintln! // same as eprint but appends a newline
//! format_args! // described below.
//! ```
//!
//! ### `write!`
//!
//! This and [`writeln!`] are two macros which are used to emit the format string
//! to a specified stream. This is used to prevent intermediate allocations of
//! format strings and instead directly write the output. Under the hood, this
//! function is actually invoking the [`write_fmt`] function defined on the
//! [`std::io::Write`] trait. Example usage is:
//!
//! ```
//! # #![allow(unused_must_use)]
//! use std::io::Write;
//! let mut w = Vec::new();
//! write!(&mut w, "Hello {}!", "world");
//! ```
//!
//! ### `print!`
//!
//! This and [`println!`] emit their output to stdout. Similarly to the [`write!`]
//! macro, the goal of these macros is to avoid intermediate allocations when
//! printing output. Example usage is:
//!
//! ```
//! print!("Hello {}!", "world");
//! println!("I have a newline {}", "character at the end");
//! ```
//! ### `eprint!`
//!
//! The [`eprint!`] and [`eprintln!`] macros are identical to
//! [`print!`] and [`println!`], respectively, except they emit their
//! output to stderr.
//!
//! ### `format_args!`
//!
//! This is a curious macro which is used to safely pass around
//! an opaque object describing the format string. This object
//! does not require any heap allocations to create, and it only
//! references information on the stack. Under the hood, all of
//! the related macros are implemented in terms of this. First
//! off, some example usage is:
//!
//! ```
//! # #![allow(unused_must_use)]
//! use std::fmt;
//! use std::io::{self, Write};
//!
//! let mut some_writer = io::stdout();
//! write!(&mut some_writer, "{}", format_args!("print with a {}", "macro"));
//!
//! fn my_fmt_fn(args: fmt::Arguments) {
//! write!(&mut io::stdout(), "{}", args);
//! }
//! my_fmt_fn(format_args!(", or a {} too", "function"));
//! ```
//!
//! The result of the [`format_args!`] macro is a value of type [`fmt::Arguments`].
//! This structure can then be passed to the [`write`] and [`format`] functions
//! inside this module in order to process the format string.
//! The goal of this macro is to even further prevent intermediate allocations
//! when dealing formatting strings.
//!
//! For example, a logging library could use the standard formatting syntax, but
//! it would internally pass around this structure until it has been determined
//! where output should go to.
//!
//! [`usize`]: ../../std/primitive.usize.html //! [`usize`]: ../../std/primitive.usize.html
//! [`isize`]: ../../std/primitive.isize.html //! [`isize`]: ../../std/primitive.isize.html
//! [`i8`]: ../../std/primitive.i8.html //! [`i8`]: ../../std/primitive.i8.html