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
//! Rust has a built in [tuple](https://doc.rust-lang.org/std/primitive.tuple.html) `(A, B, C, //! ...)` to represent a "product" of types. The language lacks a generic syntax for the converse: //! a choice among multiple types, also known as a sum type (or "coproduct") `A + B + C + ...`. //! This library provides a pattern and macro to bridge this gap. //! //! # Example //! //! ```rust //! // We can instantiate a "heterogenous" `Vec` without a custom `enum`. //! use choice::choice; //! struct A; //! struct B; //! struct C; //! let choices: Vec<choice![A, B, C]> = vec![ //! choice!(0 <- A), //! choice!(1 <- B), //! choice!(2 <- C), //! ]; //! //! // Furthermore, by implementing a trait for two `Choice` forms... //! use choice::{Choice, Never}; //! trait T {} //! impl<T1: T> T for Choice<T1, Never> {} //! impl<T1: T, T2: T> T for Choice<T1, T2> {} //! //! // ... then for types that implement the trait, any `Choice` between those types also //! // implements the trait. //! impl T for A {} //! impl T for B {} //! impl T for C {} //! fn f(t: impl T) {} //! for x in choices { //! f(x); // accepts values of type `choice![A, B, C]` //! } //! ``` //! //! # Composition Pattern //! //! The underlying pattern may be a bit counterintuitive the first time you see it. The first step //! is to use [`Choice::new`] to build a base variant on top of [`Never`]: //! //! ```rust //! use choice::{Choice, Never}; //! let no_real_choice: Choice<u64, Never> = Choice::new(42); //! ``` //! //! The `Never` type is uninhabitable and only serves to seed the pattern, so effectively we have a //! "choice" between N=1 types in the example above because an instance of the type can only hold a //! `u64`. Calling [`Choice::or`] extends a type to offer one more choice, inductively enabling a //! choice between N+1 types. //! //! ```rust //! # use choice::{Choice, Never}; //! let two_types_choice1: Choice<&'static str, Choice<u64, Never>> = //! Choice::new(42).or(); //! ``` //! //! You can build an instance of the same `Choice` type that holds the other inner type by simply //! calling `Choice::new`: //! //! ```rust //! # use choice::{Choice, Never}; //! let two_types_choice2: Choice<&'static str, Choice<u64, Never>> = //! Choice::new("Forty two"); //! ```` //! //! The above two examples share a type, so you can embed them in a collection: //! //! ```rust //! # use choice::{Choice, Never}; //! let u64_or_string_vec: Vec<Choice<&'static str, Choice<u64, Never>>> = vec![ //! Choice::new(42).or(), //! Choice::new("Forty two"), //! Choice::new(24).or(), //! Choice::new("Twenty four"), //! ]; //! ``` //! //! This pattern composes to allow additional choices: //! //! ```rust //! # use choice::{Choice, Never}; //! let many: Vec<Choice<&'static str, Choice<i8, Choice<char, Never>>>> = vec![ //! Choice::new("-INFINITY"), //! Choice::new(-1 ).or(), //! Choice::new('0' ).or().or(), //! Choice::new(1 ).or(), //! Choice::new("INFINITY" ), //! ]; //! ``` //! //! # Trait Composition //! //! Custom `enum`s serve a similar role but generally lack support for the kind of composition that //! `Choice` provides. For example, if types `A` and `B` implement trait `T`, a custom enum `AOrB` //! could also implement that trait. Unfortunately any differing choice between types would need to //! reimplement this trait, e.g. necessitating a type `AOrCOrD` for another scenario that needs to //! choose between types `A`, `C`, and `D`. //! //! By implementing trait `T` for `Choice<A: T, Never>` and `Choice<A: T, B: T>`, the trait is also //! implemented for any combination of choices. See the [Example](#example) section above or //! alternatively //! [stateright::actor::Actor](https://docs.rs/stateright/latest/stateright/actor/trait.Actor.html#foreign-impls) //! for a real-world example from another library. //! //! # Macro //! //! The [`choice!`] macro provides syntactic sugar for a type or value of the above pattern, which //! is particularly useful when there are many choices: //! //! ```rust //! # use choice::{choice, Choice, Never}; //! let x1: choice![u64, &'static str, char, String, i8] = //! choice!(2 <- 'x'); //! let x2: Choice<u64, Choice<&'static str, Choice<char, Choice<String, Choice<i8, Never>>>>> = //! Choice::new('x').or().or(); //! assert_eq!(x1, x2); //! ``` //! //! That macro also provides syntactic sugar for pattern matching on a `Choice`. Rust is unable to //! determine that the base case `Never` in uninhabited, so there is also a form to appease the //! [exhaustiveness checker](https://rustc-dev-guide.rust-lang.org/pat-exhaustive-checking.html). //! //! ```rust //! # use choice::choice; //! let c: choice![u8, char] = choice!(1 <- '?'); //! match c { //! choice!(0 -> v) => { //! panic!("Unexpected match: {}", v); //! } //! choice!(1 -> v) => { //! assert_eq!(v, '?'); //! } //! choice!(2 -> !) => { //! unreachable!(); //! } //! } //! ``` //! //! # Features //! //! Enable the `serde` feature for serialization/deserialization. #[cfg(feature = "serde")] use serde::{Deserialize, Serialize}; use std::fmt::{Display, Debug, Formatter}; /// Represents a choice between two types, which you can compose to represent a choice between more /// types -- `Choice<C, Choice<A, B>>` for instance. /// /// See the [top-level crate docs](crate) for more details. #[derive(Clone, Copy, Eq, Hash, Ord, PartialEq, PartialOrd)] #[cfg_attr(feature = "serde", derive(Deserialize, Serialize))] pub enum Choice<L, R> { /// The "left" case. L(L), /// The "right" case. R(R), } pub use Choice::{L, R}; impl<A, B> Choice<A, B> { /// Constructs a [`Choice`] between two types, where the "decision" is of the first type. pub fn new(choice: A) -> Self { L(choice) } /// Wraps an existing [`Choice`] with an additional chooseable type. pub fn or<L>(self) -> Choice<L, Choice<A, B>> { R(self) } } impl<A> Choice<A, Never> { /// Returns the "left" value, as the right value is provably uninhabited. pub fn get(&self) -> &A { match self { L(l) => l, R(_) => unreachable!(), } } } impl<A, B> Display for Choice<A, B> where A: Display, B: Display { fn fmt(&self, f: &mut Formatter<'_>) -> std::result::Result<(), std::fmt::Error> { match self { L(v) => v.fmt(f), R(v) => v.fmt(f), } } } impl<A, B> Debug for Choice<A, B> where A: Debug, B: Debug { fn fmt(&self, f: &mut Formatter<'_>) -> std::result::Result<(), std::fmt::Error> { match self { L(v) => v.fmt(f), R(v) => v.fmt(f), } } } /// Represents an uninhabited type. This is a placeholder until the built-in /// [never](https://doc.rust-lang.org/std/primitive.never.html) type is stabilized. #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)] #[cfg_attr(feature = "serde", derive(Deserialize, Serialize))] pub enum Never { } impl Display for Never { fn fmt(&self, _f: &mut Formatter<'_>) -> std::result::Result<(), std::fmt::Error> { unreachable!() } } /// Syntactic sugar for (1) a [`Choice`] among types, (2) a value inhabiting a `Choice`, and (3) /// destructuring for one of these values. /// /// Note: the `choice!(1 -> ?)`/etc forms should no longer be necessary once /// [`exhaustive_patterns`](https://doc.rust-lang.org/stable/unstable-book/language-features/exhaustive-patterns.html) /// stabilizes. /// /// # Example /// /// ```rust /// use choice::choice; /// let c: choice![u64, &'static str, char] = /// choice!(2 <- 'c'); // ^^^^ index 2 /// match c { /// choice!(0 -> v) => unreachable!("v: u64"), /// choice!(1 -> v) => unreachable!("v: &'static str"), /// choice!(2 -> v) => assert_eq!(v, 'c'), /// choice!(3 -> !) => unreachable!("v: Never"), /// } /// ``` #[macro_export] macro_rules! choice { // Type construction. [$t:ty] => ($crate::Choice<$t, $crate::Never>); [$t1:ty, $($t2:ty),+] => ($crate::Choice<$t1, choice![$($t2),+]>); // Value construction. (0 <- $x:expr) => ($crate::Choice::new($x)); (1 <- $x:expr) => (choice!(0 <- $x).or()); (2 <- $x:expr) => (choice!(1 <- $x).or()); (3 <- $x:expr) => (choice!(2 <- $x).or()); (4 <- $x:expr) => (choice!(3 <- $x).or()); (5 <- $x:expr) => (choice!(4 <- $x).or()); (6 <- $x:expr) => (choice!(5 <- $x).or()); (7 <- $x:expr) => (choice!(6 <- $x).or()); (8 <- $x:expr) => (choice!(7 <- $x).or()); (9 <- $x:expr) => (choice!(8 <- $x).or()); (10 <- $x:expr) => (choice!(9 <- $x).or()); (11 <- $x:expr) => (choice!(10 <- $x).or()); // Value destructuring for the base `Never` type. (0 -> !) => (compile_error!("Index 0 cannot be uninhabited.")); (1 -> !) => ($crate::R(_)); (2 -> !) => ($crate::R(choice!(1 -> !))); (3 -> !) => ($crate::R(choice!(2 -> !))); (4 -> !) => ($crate::R(choice!(3 -> !))); (5 -> !) => ($crate::R(choice!(4 -> !))); (6 -> !) => ($crate::R(choice!(5 -> !))); (7 -> !) => ($crate::R(choice!(6 -> !))); (8 -> !) => ($crate::R(choice!(7 -> !))); (9 -> !) => ($crate::R(choice!(8 -> !))); (10 -> !) => ($crate::R(choice!(9 -> !))); (11 -> !) => ($crate::R(choice!(10 -> !))); (12 -> !) => ($crate::R(choice!(11 -> !))); // Value destructuring for possible choices. (0 -> $v:ident) => ($crate::L($v)); (1 -> $v:ident) => ($crate::R(choice!(0 -> $v))); (2 -> $v:ident) => ($crate::R(choice!(1 -> $v))); (3 -> $v:ident) => ($crate::R(choice!(2 -> $v))); (4 -> $v:ident) => ($crate::R(choice!(3 -> $v))); (5 -> $v:ident) => ($crate::R(choice!(4 -> $v))); (6 -> $v:ident) => ($crate::R(choice!(5 -> $v))); (7 -> $v:ident) => ($crate::R(choice!(6 -> $v))); (8 -> $v:ident) => ($crate::R(choice!(7 -> $v))); (9 -> $v:ident) => ($crate::R(choice!(8 -> $v))); (10 -> $v:ident) => ($crate::R(choice!(9 -> $v))); (11 -> $v:ident) => ($crate::R(choice!(10 -> $v))); } #[cfg(test)] mod test { use super::*; #[test] fn can_compare() { let c1: choice![&'static str, char] = choice!(0 <- "a"); let c2: choice![&'static str, char] = choice!(0 <- "b"); let c3: choice![&'static str, char] = choice!(1 <- 'a'); let c4: choice![&'static str, char] = choice!(1 <- 'b'); assert!(c1 < c2); assert!(c2 < c3); // leftmost element is considered smallest assert!(c3 < c4); assert_eq!(c1, choice!(0 <- "a")); assert_eq!(c2, choice!(0 <- "b")); assert_eq!(c3, choice!(1 <- 'a')); assert_eq!(c4, choice!(1 <- 'b')); // Elements with unmatched positions are never equal, even if the values are equal. let c1: choice![char, char] = choice!(0 <- 'a'); let c2: choice![char, char] = choice!(1 <- 'a'); assert_ne!(c1, c2); } #[test] fn can_debug() { let c1: choice![&'static str, char] = choice!(0 <- "a"); let c2: choice![&'static str, char] = choice!(1 <- 'b'); assert_eq!(format!("{:?}", c1), "\"a\""); assert_eq!(format!("{:?}", c2), "'b'"); } #[test] fn can_display() { let c1: choice![&'static str, char] = choice!(0 <- "a"); let c2: choice![&'static str, char] = choice!(1 <- 'b'); assert_eq!(format!("{}", c1), "a"); assert_eq!(format!("{}", c2), "b"); } #[test] fn can_destructure() { let c1: choice![&'static str, char] = choice!(0 <- "a"); if let choice!(0 -> v) = c1 { assert_eq!(v, "a"); } else { panic!("Expected match."); } if let choice!(1 -> _v) = c1 { panic!("Unexpected match."); } let c2: choice![&'static str, char] = choice!(1 <- 'b'); match c2 { choice!(0 -> _v) => { panic!("Unexpected match."); } choice!(1 -> v) => { assert_eq!(v, 'b'); } choice!(2 -> !) => { unreachable!(); } } } #[test] fn smoke_test() { let choices: Vec<choice![u8, char, &'static str, String]> = vec![ choice!(0 <- 1), choice!(1 <- '2'), choice!(2 <- "3"), choice!(3 <- "4".to_string()), ]; assert_eq!( format!("{:?}", choices), r#"[1, '2', "3", "4"]"#); assert_eq!(choices, vec![ Choice::new(1), Choice::new('2').or(), Choice::new("3").or().or(), Choice::new("4".to_string()).or().or().or(), ]); assert_ne!(choices, vec![ choice!(0 <- 1), choice!(1 <- '2'), choice!(2 <- "three"), choice!(3 <- "4".to_string()), ]); #[derive(Debug, Eq, PartialEq)] struct A; #[derive(Debug, Eq, PartialEq)] struct B; #[derive(Debug, Eq, PartialEq)] struct C; #[derive(Debug, Eq, PartialEq)] struct D; #[derive(Debug, Eq, PartialEq)] struct E; #[derive(Debug, Eq, PartialEq)] struct F; #[derive(Debug, Eq, PartialEq)] struct G; #[derive(Debug, Eq, PartialEq)] struct H; #[derive(Debug, Eq, PartialEq)] struct I; #[derive(Debug, Eq, PartialEq)] struct J; #[derive(Debug, Eq, PartialEq)] struct K; #[derive(Debug, Eq, PartialEq)] struct L; let v: Vec<choice![A, B, C, D, E, F, G, H, I, J, K, L]> = vec![ choice!(0 <- A), choice!(1 <- B), choice!(2 <- C), choice!(3 <- D), choice!(4 <- E), choice!(5 <- F), choice!(6 <- G), choice!(7 <- H), choice!(8 <- I), choice!(9 <- J), choice!(10 <- K), choice!(11 <- L), ]; for (i, c) in v.into_iter().enumerate() { match c { choice!(0 -> v) => { assert_eq!(i, 0); assert_eq!(v, A); } choice!(1 -> v) => { assert_eq!(i, 1); assert_eq!(v, B); } choice!(2 -> v) => { assert_eq!(i, 2); assert_eq!(v, C); } choice!(3 -> v) => { assert_eq!(i, 3); assert_eq!(v, D); } choice!(4 -> v) => { assert_eq!(i, 4); assert_eq!(v, E); } choice!(5 -> v) => { assert_eq!(i, 5); assert_eq!(v, F); } choice!(6 -> v) => { assert_eq!(i, 6); assert_eq!(v, G); } choice!(7 -> v) => { assert_eq!(i, 7); assert_eq!(v, H); } choice!(8 -> v) => { assert_eq!(i, 8); assert_eq!(v, I); } choice!(9 -> v) => { assert_eq!(i, 9); assert_eq!(v, J); } choice!(10 -> v) => { assert_eq!(i, 10); assert_eq!(v, K); } choice!(11 -> v) => { assert_eq!(i, 11); assert_eq!(v, L); } choice!(12 -> !) => { unreachable!(); } } } } }