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std::fmt: reorder docs #65332

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Oct 15, 2019
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std::fmt: reorder docs #65332

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a14601e
std::fmt: move format string grammar to the bottom
RalfJung Oct 12, 2019
f727f8a
move Formatting Traits down
RalfJung Oct 12, 2019
504cc20
remove confusing and redundant subsection
RalfJung Oct 12, 2019
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366 changes: 175 additions & 191 deletions src/liballoc/fmt.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -80,24 +80,187 @@
//! arguments which have names. Like with positional parameters, it is not
//! valid to provide named parameters that are unused by the format string.
//!
//! ## Argument types
//! # 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.
//!
//! ## Fill/Alignment
//!
//! The fill character is provided normally in conjunction with the
//! [`width`](#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 the
//! following 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
//!
//! Note that alignment may not be implemented by some types. In particular, it
//! is not generally implemented for the `Debug` trait. A good way to ensure
//! padding is applied is to format your input, then use this resulting string
//! to pad your output.
//!
//! ## 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. The alternate forms are:
//! * `#?` - pretty-print the [`Debug`] formatting
//! * `#x` - precedes the argument with a `0x`
//! * `#X` - precedes the argument with a `0x`
//! * `#b` - 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 `{:08}` 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.
//! Note that padding zeroes are always placed after the sign (if any)
//! and before the digits. When used together with the `#` flag, a similar
//! rule applies: padding zeroes are inserted after the prefix but before
//! the digits.
//!
//! ## 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](#fillalignment) 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 [`usize`] in the list of
//! parameters by using the dollar syntax indicating that the second argument is
//! a [`usize`] specifying the width, for example:
//!
//! ```
//! // All of these print "Hello x !"
//! println!("Hello {:5}!", "x");
//! println!("Hello {:1$}!", "x", 5);
//! println!("Hello {1:0$}!", 5, "x");
//! println!("Hello {:width$}!", "x", width = 5);
//! ```
//!
//! Referring to an argument with the dollar syntax does not affect the "next
//! argument" counter, so it's usually a good idea to refer to arguments by
//! position, or use named arguments.
//!
//! ## Precision
//!
//! For non-numeric types, this can be considered a "maximum width". If the resulting string is
//! longer than this width, then it is truncated down to this many characters and that truncated
//! value is emitted with proper `fill`, `alignment` and `width` if those parameters are set.
//!
//! For integral types, this is ignored.
//!
//! For floating-point types, this indicates how many digits after the decimal point should be
//! printed.
//!
//! There are three possible ways to specify the desired `precision`:
//!
//! 1. An integer `.N`:
//!
//! the integer `N` itself is the precision.
//!
//! 2. An integer or name followed by dollar sign `.N$`:
//!
//! use format *argument* `N` (which must be a `usize`) as the precision.
//!
//! 3. An asterisk `.*`:
//!
//! `.*` means that this `{...}` is associated with *two* format inputs rather than one: the
//! first input holds the `usize` precision, and the second holds the value to print. Note that
//! in this case, if one uses the format string `{<arg>:<spec>.*}`, then the `<arg>` part refers
//! to the *value* to print, and the `precision` must come in the input preceding `<arg>`.
//!
//! For example, the following calls all print the same thing `Hello x is 0.01000`:
//!
//! ```
//! // Hello {arg 0 ("x")} is {arg 1 (0.01) with precision specified inline (5)}
//! println!("Hello {0} is {1:.5}", "x", 0.01);
//!
//! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision specified in arg 0 (5)}
//! println!("Hello {1} is {2:.0$}", 5, "x", 0.01);
//!
//! // Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)}
//! println!("Hello {0} is {2:.1$}", "x", 5, 0.01);
//!
//! // Hello {next arg ("x")} is {second of next two args (0.01) with precision
//! // specified in first of next two args (5)}
//! println!("Hello {} is {:.*}", "x", 5, 0.01);
//!
//! // Hello {next arg ("x")} is {arg 2 (0.01) with precision
//! // specified in its predecessor (5)}
//! println!("Hello {} is {2:.*}", "x", 5, 0.01);
//!
//! // Hello {next arg ("x")} is {arg "number" (0.01) with precision specified
//! // in arg "prec" (5)}
//! println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);
//! ```
//!
//! Each argument's type is dictated by the format string.
//! There are various parameters which require a particular type, however.
//! An example is the `{:.*}` syntax, which sets the number of decimal places
//! in floating-point types:
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Okay, I now amended this to also remove the following confusing subsection:

-//! ## Argument types
-//!
-//! Each argument's type is dictated by the format string.
-//! There are various parameters which require a particular type, however.
-//! An example is the `{:.*}` syntax, which sets the number of decimal places
-//! in floating-point types:
-//!
-//! ```
-//! let formatted_number = format!("{:.*}", 2, 1.234567);
-//!
-//! assert_eq!("1.23", formatted_number)
-//! ```
-//!
-//! If this 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 [`usize`].

//! While these:
//!
//! ```
//! let formatted_number = format!("{:.*}", 2, 1.234567);
//! println!("{}, `{name:.*}` has 3 fractional digits", "Hello", 3, name=1234.56);
//! println!("{}, `{name:.*}` has 3 characters", "Hello", 3, name="1234.56");
//! println!("{}, `{name:>8.*}` has 3 right-aligned characters", "Hello", 3, name="1234.56");
//! ```
//!
//! assert_eq!("1.23", formatted_number)
//! print two significantly different things:
//!
//! ```text
//! Hello, `1234.560` has 3 fractional digits
//! Hello, `123` has 3 characters
//! Hello, ` 123` has 3 right-aligned characters
//! ```
//!
//! If this 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 [`usize`].
//! # 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 `}}`.
//!
//! # Syntax
//!
//! To summarize, you can find the full grammar of format strings.
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This line is the only new text I wrote; everything else is just moved around.

//! 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> [ maybe-format <text> ] *
//! maybe-format := '{' '{' | '}' '}' | <format>
//! 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 := argument '$'
//! ```
//!
//! ## Formatting traits
//! # Formatting traits
//!
//! When requesting that an argument be formatted with a particular type, you
//! are actually requesting that an argument ascribes to a particular trait.
Expand Down Expand Up @@ -220,7 +383,7 @@
//! assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\"");
//! ```
//!
//! ## Related macros
//! # Related macros
//!
//! There are a number of related macros in the [`format!`] family. The ones that
//! are currently implemented are:
Expand Down Expand Up @@ -300,185 +463,6 @@
//! it would internally pass around this structure until it has been determined
//! where output should go to.
//!
//! # Syntax
//!
//! The syntax for the formatting language used is drawn from other languages,
//! so it should not be too alien. Arguments are formatted with Python-like
//! syntax, meaning that arguments are surrounded by `{}` instead of the C-like
//! `%`. The actual grammar for the formatting syntax is:
//!
//! ```text
//! format_string := <text> [ maybe-format <text> ] *
//! maybe-format := '{' '{' | '}' '}' | <format>
//! format := '{' [ argument ] [ ':' format_spec ] '}'
//! argument := integer | identifier
//!
//! format_spec := [[fill]align][sign]['#']['0'][width]['.' precision][type]
//! fill := character
//! align := '<' | '^' | '>'
//! sign := '+' | '-'
//! width := count
//! precision := count | '*'
//! type := identifier | '?' | ''
//! count := parameter | integer
//! parameter := argument '$'
//! ```
//!
//! # 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.
//!
//! ## Fill/Alignment
//!
//! The fill character is provided normally in conjunction with the
//! [`width`](#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 the
//! following 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
//!
//! Note that alignment may not be implemented by some types. In particular, it
//! is not generally implemented for the `Debug` trait. A good way to ensure
//! padding is applied is to format your input, then use this resulting string
//! to pad your output.
//!
//! ## 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. The alternate forms are:
//! * `#?` - pretty-print the [`Debug`] formatting
//! * `#x` - precedes the argument with a `0x`
//! * `#X` - precedes the argument with a `0x`
//! * `#b` - 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 `{:08}` 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.
//! Note that padding zeroes are always placed after the sign (if any)
//! and before the digits. When used together with the `#` flag, a similar
//! rule applies: padding zeroes are inserted after the prefix but before
//! the digits.
//!
//! ## 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](#fillalignment) 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 [`usize`] in the list of
//! parameters by using the dollar syntax indicating that the second argument is
//! a [`usize`] specifying the width, for example:
//!
//! ```
//! // All of these print "Hello x !"
//! println!("Hello {:5}!", "x");
//! println!("Hello {:1$}!", "x", 5);
//! println!("Hello {1:0$}!", 5, "x");
//! println!("Hello {:width$}!", "x", width = 5);
//! ```
//!
//! Referring to an argument with the dollar syntax does not affect the "next
//! argument" counter, so it's usually a good idea to refer to arguments by
//! position, or use named arguments.
//!
//! ## Precision
//!
//! For non-numeric types, this can be considered a "maximum width". If the resulting string is
//! longer than this width, then it is truncated down to this many characters and that truncated
//! value is emitted with proper `fill`, `alignment` and `width` if those parameters are set.
//!
//! For integral types, this is ignored.
//!
//! For floating-point types, this indicates how many digits after the decimal point should be
//! printed.
//!
//! There are three possible ways to specify the desired `precision`:
//!
//! 1. An integer `.N`:
//!
//! the integer `N` itself is the precision.
//!
//! 2. An integer or name followed by dollar sign `.N$`:
//!
//! use format *argument* `N` (which must be a `usize`) as the precision.
//!
//! 3. An asterisk `.*`:
//!
//! `.*` means that this `{...}` is associated with *two* format inputs rather than one: the
//! first input holds the `usize` precision, and the second holds the value to print. Note that
//! in this case, if one uses the format string `{<arg>:<spec>.*}`, then the `<arg>` part refers
//! to the *value* to print, and the `precision` must come in the input preceding `<arg>`.
//!
//! For example, the following calls all print the same thing `Hello x is 0.01000`:
//!
//! ```
//! // Hello {arg 0 ("x")} is {arg 1 (0.01) with precision specified inline (5)}
//! println!("Hello {0} is {1:.5}", "x", 0.01);
//!
//! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision specified in arg 0 (5)}
//! println!("Hello {1} is {2:.0$}", 5, "x", 0.01);
//!
//! // Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)}
//! println!("Hello {0} is {2:.1$}", "x", 5, 0.01);
//!
//! // Hello {next arg ("x")} is {second of next two args (0.01) with precision
//! // specified in first of next two args (5)}
//! println!("Hello {} is {:.*}", "x", 5, 0.01);
//!
//! // Hello {next arg ("x")} is {arg 2 (0.01) with precision
//! // specified in its predecessor (5)}
//! println!("Hello {} is {2:.*}", "x", 5, 0.01);
//!
//! // Hello {next arg ("x")} is {arg "number" (0.01) with precision specified
//! // in arg "prec" (5)}
//! println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);
//! ```
//!
//! While these:
//!
//! ```
//! println!("{}, `{name:.*}` has 3 fractional digits", "Hello", 3, name=1234.56);
//! println!("{}, `{name:.*}` has 3 characters", "Hello", 3, name="1234.56");
//! println!("{}, `{name:>8.*}` has 3 right-aligned characters", "Hello", 3, name="1234.56");
//! ```
//!
//! print two significantly different things:
//!
//! ```text
//! Hello, `1234.560` has 3 fractional digits
//! Hello, `123` has 3 characters
//! Hello, ` 123` has 3 right-aligned characters
//! ```
//!
//! # 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 `}}`.
//!
//! [`usize`]: ../../std/primitive.usize.html
//! [`isize`]: ../../std/primitive.isize.html
//! [`i8`]: ../../std/primitive.i8.html
Expand Down