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.

The functions contained in this module should not normally be used in everyday use cases of format!. The assumptions made by these functions are unsafe for all inputs, and the compiler performs a large amount of validation on the arguments to format! in order to ensure safety at runtime. While it is possible to call these functions directly, it is not recommended to do so in the general case.

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 ~str 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, {:s}!", "world"); // => ~"Hello, world!"
format!("The number is {:d}", 1); // => ~"The number is 1"
format!("{:?}", ~[3, 4]);         // => ~"~[3, 4]"
format!("{value}", value=4);      // => ~"4"
format!("{} {}", 1, 2);           // => ~"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:

format!("{1} {} {0} {}", 1, 2); // => ~"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:

identifier '=' expression

For example, the following format! expressions all use named argument:

format!("{argument}", argument = "test");       // => ~"test"
format!("{name} {}", 1, name = 2);              // => ~"2 1"
format!("{a:s} {c:d} {b:?}", a="a", b=(), c=3); // => ~"a 3 ()"

It is illegal to put positional parameters (those without names) after arguments which have names. Like positional parameters, it is illegal to provided 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. When specifying the format of an argument, however, a string like {} indicates no type. This is allowed, and if all references to one argument do not provide a type, then the format ? is used (the type's rust-representation is printed). For example, this is an invalid format string:

{0:d} {0:s}

Because the first argument is both referred to as an integer as well as a string.

Because formatting is done via traits, there is no requirement that the d format actually takes an int, but rather it simply requires a type which ascribes to the Signed formatting trait. There are various parameters which do require a particular type, however. Namely if the syntax {:.*s} is used, then the number of characters to print from the string precedes the actual string and must have the type uint. Although a uint can be printed with {:u}, it is illegal to reference an argument as such. For example, this is another invalid format string:

{:.*s} {0:u}

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 {:d} (like i8 as well as int). The current mapping of types to traits is:

• ? ⇒ Poly
• d ⇒ Signed
• i ⇒ Signed
• u ⇒ Unsigned
• b ⇒ Bool
• c ⇒ Char
• o ⇒ Octal
• x ⇒ LowerHex
• X ⇒ UpperHex
• s ⇒ String
• p ⇒ Pointer
• t ⇒ Binary
• f ⇒ Float
• e ⇒ LowerExp
• E ⇒ UpperExp
• nothing ⇒ Show

What this means is that any type of argument which implements the std::fmt::Binary trait can then be formatted with {:t}. 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 Show trait. This is one of the more commonly implemented traits when formatting a custom type.

When implementing a format trait for your own type, you will have to implement a method of the signature:

fn fmt(&self, f: &mut std::fmt::Formatter) -> fmt::Result;

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 IoError<()>). Formatting implementations should ensure that they return errors from write! correctly (propagating errors upward).

An example of implementing the formatting traits would look like:

use std::fmt;
use std::f64;

struct Vector2D {
    x: int,
    y: int,
}

impl fmt::Show for Vector2D {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // The `f.buf` value is of the type `&mut io::Writer`, which is what the
        // write! macro is expecting. Note that this formatting ignores the
        // various flags provided to format strings.
        write!(f.buf, "({}, {})", 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_bytes())
    }
}

fn main() {
    let myvector = Vector2D { x: 3, y: 4 };

    println!("{}", myvector);       // => "(3, 4)"
    println!("{:10.3t}", myvector); // => "     5.000"
}

Related macros

There are a number of related macros in the format! family. The ones that are currently implemented are:

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:

use std::io;

let mut w = io::MemWriter::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:

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;


format_args!(fmt::format, "this returns {}", "~str");
format_args!(|args| { fmt::write(my_writer, args) }, "some {}", "args");
format_args!(my_fn, "format {}", "string");

The first argument of the format_args! macro is a function (or closure) which takes one argument 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.

It is unsafe to programmatically create an instance of fmt::Arguments because the operations performed when executing a format string require the compile-time checks provided by the compiler. The format_args! macro is the only method of safely creating these structures, but they can be unsafely created with the constructor provided.

Internationalization

The formatting syntax supported by the format! extension supports internationalization by providing "methods" which execute various different outputs depending on the input. The syntax and methods provided are similar to other internationalization systems, so again nothing should seem alien. Currently two methods are supported by this extension: "select" and "plural".

Each method will execute one of a number of clauses, and then the value of the clause will become what's the result of the argument's format. Inside of the cases, nested argument strings may be provided, but all formatting arguments must not be done through implicit positional means. All arguments inside of each case of a method must be explicitly selected by their name or their integer position.

Furthermore, whenever a case is running, the special character # can be used to reference the string value of the argument which was selected upon. As an example:

format!("{0, select, other{#}}", "hello"); // => ~"hello"

This example is the equivalent of {0:s} essentially.

Select

The select method is a switch over a &str parameter, and the parameter must be of the type &str. An example of the syntax is:

{0, select, male{...} female{...} other{...}}

Breaking this down, the 0-th argument is selected upon with the select method, and then a number of cases follow. Each case is preceded by an identifier which is the match-clause to execute the given arm. In this case, there are two explicit cases, male and female. The case will be executed if the string argument provided is an exact match to the case selected.

The other case is also a required case for all select methods. This arm will be executed if none of the other arms matched the word being selected over.

Plural

The plural method is a switch statement over a uint parameter, and the parameter must be a uint. A plural method in its full glory can be specified as:

{0, plural, offset=1 =1{...} two{...} many{...} other{...}}

To break this down, the first 0 indicates that this method is selecting over the value of the first positional parameter to the format string. Next, the plural method is being executed. An optionally-supplied offset is then given which indicates a number to subtract from argument 0 when matching. This is then followed by a list of cases.

Each case is allowed to supply a specific value to match upon with the syntax =N. This case is executed if the value at argument 0 matches N exactly, without taking the offset into account. A case may also be specified by one of five keywords: zero, one, two, few, and many. These cases are matched on after argument 0 has the offset taken into account. Currently the definitions of many and few are hardcoded, but they are in theory defined by the current locale.

Finally, all plural methods must have an other case supplied which will be executed if none of the other cases match.

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:

format_string := <text> [ format <text> ] *
format := '{' [ argument ] [ ':' format_spec ] [ ',' function_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 '$'

function_spec := plural | select
select := 'select' ',' ( identifier arm ) *
plural := 'plural' ',' [ 'offset:' integer ] ( selector arm ) *
selector := '=' integer | keyword
keyword := 'zero' | 'one' | 'two' | 'few' | 'many' | 'other'
arm := '{' format_string '}'

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 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 {, }, or # may be included in a string by preceding them with the \ character. Since \ is already an escape character in Rust strings, a string literal using this escape will look like "\\{".