squiid/rust
1
0
Fork 0
Code Issues Pull requests Projects Activity

Explain what ifmt! is all about

Browse source
This commit is contained in:
Alex Crichton 2013-08-10 18:21:34 -07:00
parent 1f6afa887b
commit 27b4d104c8
1 changed files with 301 additions and 0 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

301
src/libstd/fmt/mod.rs
View file

@ -8,6 +8,307 @@
// option. This file may not be copied, modified, or distributed
// except according to those terms.
/**!
# The Formatting Module
This module contains the runtime support for the `ifmt!` 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 `ifmt!`. The assumptions made by these functions are unsafe for all
inputs, and the compiler performs a large amount of validation on the arguments
to `ifmt!` 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 `ifmt!` macro is intended to be familiar to those coming from C's
printf/sprintf functions or Python's `str.format` function. In its current
revision, the `ifmt!` 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 `ifmt!` extension are:
~~~{.rust}
ifmt!("Hello") // => ~"Hello"
ifmt!("Hello, {:s}!", "world") // => ~"Hello, world!"
ifmt!("The number is {:d}", 1) // => ~"The number is 1"
ifmt!("{}", ~[3, 4]) // => ~"~[3, 4]"
ifmt!("{value}", value=4) // => ~"4"
ifmt!("{} {}", 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.
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 `ifmt!` 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
~~~
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 sytnax `{:.*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
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. Again, the default formatting type (if no other is specified)
is `?` which is defined for all types by default.
When implementing a format trait for your own time, you will have to implement a
method of the signature:
~~~
fn fmt(value: &T, f: &mut std::fmt::Formatter);
~~~
Your type will be passed by-reference in `value`, 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.
## Internationalization
The formatting syntax supported by the `ifmt!` extension supports
internationalization by providing "methods" which execute various differnet
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:
~~~
ifmt!("{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.
*/
use prelude::*;
use cast;