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reorder fmt docs for more clarity #65513

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Oct 19, 2019
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reorder fmt docs for more clarity #65513

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4e6efe4
reorder fmt docs for more clarity
RalfJung Oct 17, 2019
c0b7e76
example for padding any format
RalfJung Oct 17, 2019
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111 changes: 67 additions & 44 deletions src/liballoc/fmt.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -86,27 +86,74 @@
//! parameters (corresponding to `format_spec` in the syntax above). These
//! parameters affect the string representation of what's being formatted.
//!
//! ## Width
//!
//! ```
//! // All of these print "Hello x !"
//! println!("Hello {:5}!", "x");
//! println!("Hello {:1$}!", "x", 5);
//! println!("Hello {1:0$}!", 5, "x");
//! println!("Hello {:width$}!", "x", width = 5);
//! ```
//!
//! This is a parameter for the "minimum width" that the format should take up.
//! If the value's string does not fill up this many characters, then the
//! padding specified by fill/alignment will be used to take up the required
//! space (see below).
//!
//! The value for the width can also be provided as a [`usize`] in the list of
//! parameters by adding a postfix `$`, indicating that the second argument is
//! a [`usize`] specifying the width.
//!
//! 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.
//!
//! ## 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:
//! ```
//! assert_eq!(format!("Hello {:<5}!", "x"), "Hello x !");
//! assert_eq!(format!("Hello {:-<5}!", "x"), "Hello x----!");
//! assert_eq!(format!("Hello {:^5}!", "x"), "Hello x !");
//! assert_eq!(format!("Hello {:>5}!", "x"), "Hello x!");
//! ```
//!
//! * `<` - the argument is left-aligned in `width` columns
//! * `^` - the argument is center-aligned in `width` columns
//! * `>` - the argument is right-aligned in `width` columns
//! The optional fill character and alignment is provided normally in conjunction with the
//! [`width`](#width) parameter. It must be defined before `width`, right after the `:`.
//! This indicates that if the value being formatted is smaller than
//! `width` some extra characters will be printed around it.
//! Filling comes in the following variants for different alignments:
//!
//! * `[fill]<` - the argument is left-aligned in `width` columns
//! * `[fill]^` - the argument is center-aligned in `width` columns
//! * `[fill]>` - the argument is right-aligned in `width` columns
//!
//! 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 (see below) is specified for numerics, then the implicit fill character is
//! `0`.
//!
//! 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
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FWIW, I do not understand this paragraph. Do they mean nested format!?

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Yes, essentially -- i.e., strings definitely implement padding. So if padding is desired, a guaranteed way to obtain it is to format a string (and that string can be obtained via formatting).

I agree this paragraph is pretty confusing.

//! padding is applied is to format your input, then use this resulting string
//! to pad your output.
//! padding is applied is to format your input, then pad this resulting string
//! to obtain your output:
//!
//! ```
//! println!("Hello {:^15}!", format!("{:?}", Some("hi"))); // => "Hello Some("hi") !"
//! ```
//!
//! ## Sign/`#`/`0`
//!
//! These can all be interpreted as flags for a particular formatter.
//! ```
//! assert_eq!(format!("Hello {:+}!", 5), "Hello +5!");
//! assert_eq!(format!("{:#x}!", 27), "0x1b!");
//! assert_eq!(format!("Hello {:05}!", 5), "Hello 00005!");
//! assert_eq!(format!("Hello {:05}!", -5), "Hello -0005!");
//! assert_eq!(format!("{:#010x}!", 27), "0x0000001b!");
//! ```
//!
//! These are all flags altering the behavior of the formatter.
//!
//! * `+` - This is intended for numeric types and indicates that the sign
//! should always be printed. Positive signs are never printed by
Expand All @@ -121,44 +168,15 @@
//! * `#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
//! * `0` - This is used to indicate for integer formats that the padding to `width` 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.
//! the digits. The prefix is included in the total width.
//!
//! ## Precision
//!
Expand Down Expand Up @@ -235,9 +253,14 @@
//! them with the same character. For example, the `{` character is escaped with
//! `{{` and the `}` character is escaped with `}}`.
//!
//! ```
//! assert_eq!(format!("Hello {{}}"), "Hello {}");
//! assert_eq!(format!("{{ Hello"), "{ Hello");
//! ```
//!
//! # Syntax
//!
//! To summarize, you can find the full grammar of format strings.
//! To summarize, here you can find the full grammar of format strings.
//! 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
Expand Down