rust/src/libstd/fmt.rs

// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//
// ignore-lexer-test FIXME #15679

//! Utilities for formatting and printing strings
//!
//! This module contains the runtime support for the `format!` syntax extension.
//! This macro is implemented in the compiler to emit calls to this module in
//! order to format arguments at runtime into strings and streams.
//!
//! ## Usage
//!
//! The `format!` macro is intended to be familiar to those coming from C's
//! printf/fprintf functions or Python's `str.format` function. In its current
//! revision, the `format!` macro returns a `String` type which is the result of
//! the formatting. In the future it will also be able to pass in a stream to
//! format arguments directly while performing minimal allocations.
//!
//! Some examples of the `format!` extension are:
//!
//! ```
//! format!("Hello");                  // => "Hello"
//! format!("Hello, {}!", "world");    // => "Hello, world!"
//! format!("The number is {}", 1i);   // => "The number is 1"
//! format!("{:?}", (3i, 4i));         // => "(3i, 4i)"
//! format!("{value}", value=4i);      // => "4"
//! format!("{} {}", 1i, 2u);          // => "1 2"
//! ```
//!
//! From these, you can see that the first argument is a format string. It is
//! required by the compiler for this to be a string literal; it cannot be a
//! variable passed in (in order to perform validity checking). The compiler
//! will then parse the format string and determine if the list of arguments
//! provided is suitable to pass to this format string.
//!
//! ### Positional parameters
//!
//! Each formatting argument is allowed to specify which value argument it's
//! referencing, and if omitted it is assumed to be "the next argument". For
//! example, the format string `{} {} {}` would take three parameters, and they
//! would be formatted in the same order as they're given. The format string
//! `{2} {1} {0}`, however, would format arguments in reverse order.
//!
//! Things can get a little tricky once you start intermingling the two types of
//! positional specifiers. The "next argument" specifier can be thought of as an
//! iterator over the argument. Each time a "next argument" specifier is seen,
//! the iterator advances. This leads to behavior like this:
//!
//! ```rust
//! format!("{1} {} {0} {}", 1i, 2i); // => "2 1 1 2"
//! ```
//!
//! The internal iterator over the argument has not been advanced by the time
//! the first `{}` is seen, so it prints the first argument. Then upon reaching
//! the second `{}`, the iterator has advanced forward to the second argument.
//! Essentially, parameters which explicitly name their argument do not affect
//! parameters which do not name an argument in terms of positional specifiers.
//!
//! A format string is required to use all of its arguments, otherwise it is a
//! compile-time error. You may refer to the same argument more than once in the
//! format string, although it must always be referred to with the same type.
//!
//! ### Named parameters
//!
//! Rust itself does not have a Python-like equivalent of named parameters to a
//! function, but the `format!` macro is a syntax extension which allows it to
//! leverage named parameters. Named parameters are listed at the end of the
//! argument list and have the syntax:
//!
//! ```text
//! identifier '=' expression
//! ```
//!
//! For example, the following `format!` expressions all use named argument:
//!
//! ```
//! format!("{argument}", argument = "test");   // => "test"
//! format!("{name} {}", 1i, name = 2i);        // => "2 1"
//! format!("{a} {c} {b}", a="a", b='b', c=3i);  // => "a 3 b"
//! ```
//!
//! It is illegal to put positional parameters (those without names) after
//! arguments which have names. Like with positional parameters, it is illegal
//! to provide named parameters that are unused by the format string.
//!
//! ### Argument types
//!
//! Each argument's type is dictated by the format string. It is a requirement
//! that every argument is only ever referred to by one type. For example, this
//! is an invalid format string:
//!
//! ```text
//! {0:x} {0:o}
//! ```
//!
//! This is invalid because the first argument is both referred to as a
//! hexadecimal as well as an
//! octal.
//!
//! There are various parameters which do require a particular type, however.
//! Namely if the syntax `{:.*}` is used, then the number of characters to print
//! precedes the actual object being formatted, and the number of characters
//! must have the type `uint`. Although a `uint` can be printed with `{}`, it is
//! illegal to reference an argument as such. For example this is another
//! invalid format string:
//!
//! ```text
//! {:.*} {0}
//! ```
//!
//! ### 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 `int`).  The current mapping of types to traits is:
//!
//! * *nothing* ⇒ `Display`
//! * `?` ⇒ `Debug`
//! * `o` ⇒ `Octal`
//! * `x` ⇒ `LowerHex`
//! * `X` ⇒ `UpperHex`
//! * `p` ⇒ `Pointer`
//! * `b` ⇒ `Binary`
//! * `e` ⇒ `LowerExp`
//! * `E` ⇒ `UpperExp`
//!
//! What this means is that any type of argument which implements the
//! `std::fmt::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:
//!
//! ```rust
//! # use std::fmt;
//! # struct Foo; // our custom type
//! # impl fmt::Display for Foo {
//! fn fmt(&self, f: &mut std::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
//! typedef to `Result<(), IoError>` (also known as `IoResult<()>`). Formatting
//! implementations should ensure that they return errors from `write!`
//! correctly (propagating errors upward).
//!
//! An example of implementing the formatting traits would look
//! like:
//!
//! ```rust
//! use std::fmt;
//! use std::f64;
//! use std::num::Float;
//!
//! #[derive(Debug)]
//! struct Vector2D {
//!     x: int,
//!     y: int,
//! }
//!
//! impl fmt::Display for Vector2D {
//!     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//!         // The `f` value implements the `Writer` 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 = f64::to_str_exact(magnitude, decimals);
//!         f.pad_integral(true, "", string.as_slice())
//!     }
//! }
//!
//! fn main() {
//!     let myvector = Vector2D { x: 3, y: 4 };
//!
//!     println!("{}", myvector);       // => "(3, 4)"
//!     println!("{:?}", myvector);     // => "Vector2D {x: 3i, y:4i}"
//!     println!("{:10.3b}", myvector); // => "     5.000"
//! }
//! ```
//!
//! #### fmt::Display vs fmt::Debug
//!
//! These two formatting traits have distinct purposes:
//!
//! - `fmt::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` 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!("{} {:?}", 3i32, 4i32), "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
//! format!      // described above
//! write!       // first argument is a &mut io::Writer, 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
//! 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` function defined in this module.
//! Example usage is:
//!
//! ```rust
//! # #![allow(unused_must_use)]
//! use std::io;
//!
//! let mut w = Vec::new();
//! write!(&mut w as &mut io::Writer, "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:
//!
//! ```rust
//! print!("Hello {}!", "world");
//! println!("I have a newline {}", "character at the end");
//! ```
//!
//! #### `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:
//!
//! ```
//! use std::fmt;
//! use std::io;
//!
//! fmt::format(format_args!("this returns {}", "String"));
//!
//! let some_writer: &mut io::Writer = &mut io::stdout();
//! write!(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.
//!
//! ## Syntax
//!
//! 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
//! syntax, meaning that arguments are surrounded by `{}` instead of the C-like
//! `%`. The actual grammar for the formatting syntax is:
//!
//! ```text
//! format_string := <text> [ format <text> ] *
//! format := '{' [ argument ] [ ':' format_spec ] '}'
//! argument := integer | identifier
//!
//! format_spec := [[fill]align][sign]['#'][0][width]['.' precision][type]
//! fill := character
//! align := '<' | '^' | '>'
//! sign := '+' | '-'
//! width := count
//! precision := count | '*'
//! type := identifier | ''
//! count := parameter | integer
//! parameter := integer '$'
//! ```
//!
//! ## Formatting Parameters
//!
//! Each argument being formatted can be transformed by a number of formatting
//! parameters (corresponding to `format_spec` in the syntax above). These
//! parameters affect the string representation of what's being formatted. This
//! syntax draws heavily from Python's, so it may seem a bit familiar.
//!
//! ### Fill/Alignment
//!
//! The fill character is provided normally in conjunction with the `width`
//! parameter. This indicates that if the value being formatted is smaller than
//! `width` some extra characters will be printed around it. The extra
//! characters are specified by `fill`, and the alignment can be one of two
//! options:
//!
//! * `<` - the argument is left-aligned in `width` columns
//! * `^` - the argument is center-aligned in `width` columns
//! * `>` - the argument is right-aligned in `width` columns
//!
//! ### Sign/#/0
//!
//! These can all be interpreted as flags for a particular formatter.
//!
//! * '+' - This is intended for numeric types and indicates that the sign
//!         should always be printed. Positive signs are never printed by
//!         default, and the negative sign is only printed by default for the
//!         `Signed` trait. This flag indicates that the correct sign (+ or -)
//!         should always be printed.
//! * '-' - Currently not used
//! * '#' - This flag is indicates that the "alternate" form of printing should
//!         be used. By default, this only applies to the integer formatting
//!         traits and performs like:
//!     * `x` - precedes the argument with a "0x"
//!     * `X` - precedes the argument with a "0x"
//!     * `t` - precedes the argument with a "0b"
//!     * `o` - precedes the argument with a "0o"
//! * '0' - This is used to indicate for integer formats that the padding should
//!         both be done with a `0` character as well as be sign-aware. A format
//!         like `{:08d}` would yield `00000001` for the integer `1`, while the
//!         same format would yield `-0000001` for the integer `-1`. Notice that
//!         the negative version has one fewer zero than the positive version.
//!
//! ### Width
//!
//! This is a parameter for the "minimum width" that the format should take up.
//! If the value's string does not fill up this many characters, then the
//! padding specified by fill/alignment will be used to take up the required
//! space.
//!
//! The default fill/alignment for non-numerics is a space and left-aligned. The
//! defaults for numeric formatters is also a space but with right-alignment. If
//! the '0' flag is specified for numerics, then the implicit fill character is
//! '0'.
//!
//! The value for the width can also be provided as a `uint` in the list of
//! parameters by using the `2$` syntax indicating that the second argument is a
//! `uint` specifying the width.
//!
//! ### Precision
//!
//! For non-numeric types, this can be considered a "maximum width". If the
//! resulting string is longer than this width, then it is truncated down to
//! this many characters and only those are emitted.
//!
//! For integral types, this has no meaning currently.
//!
//! For floating-point types, this indicates how many digits after the decimal
//! point should be printed.
//!
//! ## Escaping
//!
//! The literal characters `{` and `}` may be included in a string by preceding
//! them with the same character. For example, the `{` character is escaped with
//! `{{` and the `}` character is escaped with `}}`.

#![unstable]

use string;

pub use core::fmt::{Formatter, Result, Writer, rt};
pub use core::fmt::{Show, String, Octal, Binary};
pub use core::fmt::{Display, Debug};
pub use core::fmt::{LowerHex, UpperHex, Pointer};
pub use core::fmt::{LowerExp, UpperExp};
pub use core::fmt::Error;
pub use core::fmt::{Argument, Arguments, write, radix, Radix, RadixFmt};

#[doc(hidden)]
pub use core::fmt::{argument, argumentuint};

/// The format function takes a precompiled format string and a list of
/// arguments, to return the resulting formatted string.
///
/// # Arguments
///
///   * args - a structure of arguments generated via the `format_args!` macro.
///
/// # Example
///
/// ```rust
/// use std::fmt;
///
/// let s = fmt::format(format_args!("Hello, {}!", "world"));
/// assert_eq!(s, "Hello, world!".to_string());
/// ```
#[unstable = "this is an implementation detail of format! and should not \
                  be called directly"]
pub fn format(args: Arguments) -> string::String {
    let mut output = string::String::new();
    let _ = write!(&mut output, "{}", args);
    output
}