1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499
/*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at https://mozilla.org/MPL/2.0/.
*/
// Copyright 2019-2022 Nathan West
#![no_std]
/*!
[`lazy_format!`] is a [`format!`]-style macro which lazily formats its
arguments. That is, rather than immediatly formatting them into a [`String`]
(which is what [`format!`] does), it captures its arguments and returns an
opaque struct with a [`Display`] implementation, so that the actual formatting
can happen directly into its final destination buffer (such as a file or
string).
```
use std::fmt::Display;
use lazy_format::prelude::*;
// NOTE: This is profoundly insecure and you should never actually
// render HTML without escape guards, code injection prevention, etc.
fn html_tag<'a>(tag: &'a str, content: impl Display + 'a) -> impl Display + 'a {
lazy_format!("<{tag}>{content}</{tag}>")
}
let result = html_tag("div", html_tag("p", "Hello, World!")).to_string();
assert_eq!(result, "<div><p>Hello, World!</p></div>");
```
[`format!`]: https://doc.rust-lang.org/std/macro.format.html
[`Display`]: https://doc.rust-lang.org/std/fmt/trait.Display.html
[`String`]: https://doc.rust-lang.org/std/string/struct.String.html
*/
/**
Smarter write macro. Encodes some common patterns, such as writing an empty
string being a no-op. Used in the more complex lazy-format operations, like
conditionals, where writing only strings or empty strings, is common.
*/
#[macro_export]
#[doc(hidden)]
macro_rules! write {
($dest:expr, "" $(,)? ) => { ::core::fmt::Result::Ok(()) };
($dest:expr, $pattern:literal $(,)? ) => {{
enum Style {
Empty,
Plain,
Format,
}
match {
// We want this to be const so that we can guarantee it's computed
// at compile time. Need to put the `STYLE` global in a nested
// scope because otherwise it leaks out
const STYLE: Style = match $pattern.as_bytes().split_first() {
::core::option::Option::None => Style::Empty,
::core::option::Option::Some((&(b'}' | b'{'), _)) => Style::Format,
::core::option::Option::Some((_, mut s)) => loop {
s = match s.split_first() {
None => break Style::Plain,
Some((&(b'}' | b'{'), _)) => break Style::Format,
Some((_, s)) => s,
};
}
};
STYLE
} {
Style::Empty => ::core::fmt::Result::Ok(()),
Style::Plain => ::core::fmt::Write::write_str($dest, $pattern),
Style::Format => ::core::fmt::Write::write_fmt($dest, ::core::format_args!($pattern)),
}
}};
($dest:expr, $pattern:literal, $($args:tt)+ ) => {
::core::fmt::Write::write_fmt($dest, ::core::format_args!($pattern, $($args)+))
};
}
/**
Helper macro for common formatting shortcuts. In a few places in lazy_format,
it's permitted to write either `lazy_format!(if cond => "foo")` or
`lazy_format!(if cond => ("value: {}", value))`. This macro takes care of
handling both cases.
*/
#[macro_export]
#[doc(hidden)]
macro_rules! write_tt {
($dest:expr, $pattern:literal) => { $crate::write!($dest, $pattern) };
($dest:expr, ($pattern:literal $($args:tt)*)) => { $crate::write!($dest, $pattern $($args)*) };
}
/// Test that an empty format string succeeds unconditionally.
#[test]
fn test_write_tt_empty_pattern() {
use core::fmt;
struct BadDest;
impl fmt::Write for BadDest {
fn write_str(&mut self, _s: &str) -> fmt::Result {
Err(fmt::Error)
}
}
let x = 10;
write_tt!(&mut BadDest, "").unwrap();
write_tt!(&mut BadDest, ("")).unwrap();
write_tt!(&mut BadDest, ("",)).unwrap();
write_tt!(&mut BadDest, "Plain String").unwrap_err();
write_tt!(&mut BadDest, "Formatted String: {x}").unwrap_err();
}
#[test]
fn test_write_string_pattern() {
use core::fmt;
struct WeirdDest;
impl fmt::Write for WeirdDest {
fn write_str(&mut self, _s: &str) -> fmt::Result {
Ok(())
}
fn write_fmt(&mut self, _args: fmt::Arguments<'_>) -> fmt::Result {
Err(fmt::Error)
}
}
let x = 10;
write_tt!(&mut WeirdDest, "Plain String").unwrap();
write_tt!(&mut WeirdDest, "Formatted String: {x}").unwrap_err();
}
/**
Low level constructor for lazy format instances. Create a lazy formatter with a
custom closure as its [`Display`] implementation, for complete control over
formatting behavior at write time.
[`make_lazy_format!`] is the low-level constructor for lazy format instances. It
is completely customizable, insofar as it allows you to create a custom
[`Display::fmt`] implementation at the call site.
[`make_lazy_format!`] takes a closure as an argument, and creates a [`Display`]
struct that captures the local environment in a closure and uses it as the
formatting function.
# Example:
```
use std::fmt::Display;
use lazy_format::make_lazy_format;
let data = vec![1, 2, 3, 4, 5];
let comma_separated = make_lazy_format!(|f| {
let mut iter = data.iter();
match iter.next() {
None => Ok(()),
Some(first) => {
write!(f, "{}", first)?;
iter.try_for_each(|value| write!(f, ", {}", value))
}
}
});
let result = comma_separated.to_string();
assert_eq!(result, "1, 2, 3, 4, 5");
```
[`Display`]: https://doc.rust-lang.org/std/fmt/trait.Display.html
[`Display::fmt`]: https://doc.rust-lang.org/core/fmt/trait.Display.html#tymethod.fmt
*/
#[macro_export]
macro_rules! make_lazy_format {
(|$fmt:ident| $write:expr) => {{
#[derive(Clone, Copy)]
struct LazyFormat<F: Fn(&mut ::core::fmt::Formatter) -> ::core::fmt::Result>(F);
// TODO: customize Debug impl for semi_lazy_format to include value
impl<F: Fn(&mut ::core::fmt::Formatter) -> ::core::fmt::Result> ::core::fmt::Debug
for LazyFormat<F>
{
#[inline]
fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
f.write_str(concat!(
"make_lazy_format!(",
stringify!(|$fmt| $write),
")"
))
}
}
impl<F: Fn(&mut ::core::fmt::Formatter) -> ::core::fmt::Result> ::core::fmt::Display
for LazyFormat<F>
{
#[inline]
fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
(self.0)(f)
}
}
LazyFormat(move |$fmt: &mut ::core::fmt::Formatter| -> ::core::fmt::Result { $write })
}};
}
/**
Lazily format something. Essentially the same as [`format!`], except that
instead of formatting its arguments to a string, it captures them in an opaque
struct, which can be formatted later. This allows you to build up formatting
operations without any intermediary allocations or extra formatting calls. Also
supports lazy conditional and looping constructs.
The return value of this macro is left deliberately unspecified and
undocumented. The most important this about it is its [`Display`]
implementation, which executes the deferred formatting operation. It also
provides a [`Debug`] implementation, which simply prints the
[`lazy_format!`]`(...)` call without evaluating any of its arguments, as well
as [`Clone`] and [`Copy`] if those traits are available in the captured
context.
Note that this macro is completely lazy; it captures the expressions to
be formatted in the struct and doesn't evaluate them until the struct is
actually written to a [`String`] or [`File`] or or other writable destination.
This means that the argument expression will be evaluated *every* time the
instance is written, which may not be what you want; be sure to eagerly perform
any 1-time calculations you want to before calling `lazy_format!`.
# Basic example:
```
use std::fmt::Display;
use lazy_format::lazy_format;
fn get_hello() -> String {
String::from("Hello")
}
fn get_world() -> String {
String::from("World")
}
fn hello_world() -> impl Display {
lazy_format!("{}, {w}!", get_hello(), w = get_world())
}
let result = hello_world();
// get_hello and get_world aren't called until the object is
// formatted into a String.
let result_str = result.to_string();
assert_eq!(result_str, "Hello, World!");
```
Just like with regular formatting, `lazy_format` can automatically, implicitly
capture named parameters:
```
use std::mem::{size_of_val, size_of};
use lazy_format::lazy_format;
let a = 10;
let b = 20;
let result = lazy_format!("{a} {b}");
assert_eq!(size_of_val(&result), size_of::<i32>() * 2);
assert_eq!(result.to_string(), "10 20");
```
# Demonstation of lazy capturing:
```
use std::fmt::Display;
use std::mem::{size_of_val, size_of};
use lazy_format::lazy_format;
fn get_formatted() -> impl Display {
let a: isize = 10;
let b: isize = 15;
lazy_format!("10 + 15: {v}, again: {v}", v = (a + b))
}
let result = get_formatted();
// The result captures 2 isize values (a and b) from get_formatted.
assert_eq!(size_of_val(&result), size_of::<isize>() * 2);
let result_str = result.to_string();
assert_eq!(result_str, "10 + 15: 25, again: 25");
```
# Conditional formatting
`lazy_format!` supports conditional formatting with `match`- or `if`-
style syntax. When doing a conditional format, add the formatting pattern
and arguments directly into the `match` arms or `if` blocks, rather than
code; this allows conditional formatting to still be captured in a single
static type.
## `match` conditional example:
```
use std::fmt::Display;
use lazy_format::lazy_format;
fn get_number(num: usize) -> impl Display {
// Note that the parenthesis in the match conditional are required,
// due to limitations in Rust's macro parsing (can't follow an
// expression with `{}`)
lazy_format!(match (num) {
0 => "Zero",
1 => "One",
2 => "Two",
3 => "Three",
4 | 5 => "Four or five",
value if value % 2 == 0 => ("A large even number: {}", value),
value => "An unrecognized number: {value}",
})
}
assert_eq!(get_number(0).to_string(), "Zero");
assert_eq!(get_number(1).to_string(), "One");
assert_eq!(get_number(2).to_string(), "Two");
assert_eq!(get_number(3).to_string(), "Three");
assert_eq!(get_number(4).to_string(), "Four or five");
assert_eq!(get_number(5).to_string(), "Four or five");
assert_eq!(get_number(6).to_string(), "A large even number: 6");
assert_eq!(get_number(7).to_string(), "An unrecognized number: 7");
```
## `if` conditional example:
```
use std::fmt::Display;
use lazy_format::lazy_format;
fn describe_number(value: isize) -> impl Display {
lazy_format!(
if value < 0 => ("A negative number: {}", value)
else if value % 3 == 0 => ("A number divisible by 3: {}", value)
else if value % 2 == 1 => ("An odd number: {}", value)
else => "Some other kind of number"
)
}
assert_eq!(describe_number(-2).to_string(), "A negative number: -2");
assert_eq!(describe_number(-1).to_string(), "A negative number: -1");
assert_eq!(describe_number(0).to_string(), "A number divisible by 3: 0");
assert_eq!(describe_number(1).to_string(), "An odd number: 1");
assert_eq!(describe_number(2).to_string(), "Some other kind of number");
assert_eq!(describe_number(3).to_string(), "A number divisible by 3: 3");
```
`if` formatters are allowed to exclude the final `else` branch, in which
case `lazy_format` will simply write an empty string:
```
use std::fmt::Display;
use lazy_format::lazy_format;
fn only_evens(value: i32) -> impl Display {
lazy_format!(if value % 2 == 0 => ("An even number: {}", value))
}
assert_eq!(only_evens(10).to_string(), "An even number: 10");
assert_eq!(only_evens(5).to_string(), "");
```
## `if let` conditional example:
```
use std::fmt::Display;
use lazy_format::lazy_format;
fn describe_optional_number(value: Option<isize>) -> impl Display {
lazy_format!(
if let Some(10) = value => "It's ten!"
else if let Some(3) | Some(4) = value => "It's three or four!"
else if let | Some(0) = value => "It's zero!"
else if let Some(x) = value => ("It's some other value: {}", x)
else => "It's not a number!"
)
}
assert_eq!(describe_optional_number(Some(10)).to_string(), "It's ten!");
assert_eq!(describe_optional_number(Some(3)).to_string(), "It's three or four!");
assert_eq!(describe_optional_number(Some(4)).to_string(), "It's three or four!");
assert_eq!(describe_optional_number(Some(0)).to_string(), "It's zero!");
assert_eq!(describe_optional_number(Some(5)).to_string(), "It's some other value: 5");
assert_eq!(describe_optional_number(None).to_string(), "It's not a number!");
```
# Looping formatting
`lazy_format!` supports formatting elements in a collection with a loop:
```
use std::fmt::Display;
use lazy_format::lazy_format;
let list = vec![1i32, 2, 3, 4];
let list_ref = &list;
// Format each element in the iterable without additional arguments to `format_args`
let simple_semicolons = lazy_format!("{v}; " for v in list_ref.iter().map(|x| x - 1));
assert_eq!(simple_semicolons.to_string(), "0; 1; 2; 3; ");
// Perform a full format with additional arguments on each element in the iterable.
let header = "Value";
let full_format = lazy_format!(("{}: {}; ", header, v) for v in list_ref);
assert_eq!(full_format.to_string(), "Value: 1; Value: 2; Value: 3; Value: 4; ");
```
Note that these looping formatters are not suitable for doing something like
a comma separated list, since they'll apply the formatting to all elements.
For a lazy string joining library, which only inserts separators between
elements in a list, check out [joinery](/joinery).
[`format!`]: https://doc.rust-lang.org/std/macro.format.html
[`Display`]: https://doc.rust-lang.org/std/fmt/trait.Display.html
[`Debug`]: https://doc.rust-lang.org/std/fmt/trait.Debug.html
[`String`]: https://doc.rust-lang.org/std/string/struct.String.html
[`File`]: https://doc.rust-lang.org/std/fs/struct.File.html
*/
#[macro_export]
macro_rules! lazy_format {
// Basic lazy format: collect $args and format via `$pattern` when writing
// to a destination
($pattern:literal $(, $($args:tt)*)?) => {
$crate::make_lazy_format!(|f| $crate::write!(f, $pattern $(, $($args)*)?))
};
// Conditional lazy format: evaluate a match expression and format based on
// the matching arm
(match ($condition:expr) {
$($(
$match_pattern:pat
$(if $guard:expr)?
=> $output:tt
),+ $(,)?)?
}) => {
$crate::make_lazy_format!(|f| match $condition {
$($(
$match_pattern
$(if $guard)?
=> $crate::write_tt!(f, $output),
)+)?
})
};
// Conditional pattern lazy format: evaluate
// Conditional lazy format: evaluate an if / else if / else expression and
// format based on the successful branch
(
if $(let $match:pat = )? $condition:expr => $output:tt
$(else if $(let $elseif_match:pat = )? $elseif_condition:expr => $elseif_output:tt)*
$(else => $else_output:tt)?
) => {
$crate::make_lazy_format!(|f|
if $(let $match = )? $condition {
$crate::write_tt!(f, $output)
}
$(else if $(let $elseif_match = )? $elseif_condition {
$crate::write_tt!(f, $elseif_output)
})*
$(else if true {
$crate::write_tt!(f, $else_output)
})?
else {
::core::fmt::Result::Ok(())
}
)
};
// Looping formatter: format each `$item` in `$collection` with the format
// arguments
($output:tt for $item:pat in $collection:expr) => {
$crate::make_lazy_format!(|f| {
let mut iter = ::core::iter::IntoIterator::into_iter($collection);
::core::iter::Iterator::try_for_each(&mut iter, |$item| $crate::write_tt!(f, $output))
})
};
}
pub mod prelude {
pub use crate::{lazy_format, make_lazy_format};
}