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
//! # `do-notation`, the monadic `do` notation brought to Rust. //! //! This crate provides the `m!` macro, which provides the Haskell monadic syntactic sugar `do`. //! //! > Note: it is not possible to use the `do!` syntax as `do` is a reserved keyword in Rust. //! //! The syntax is very similar to what you find in Haskell: //! //! - You use the `m!` macro; in Haskell, you use the `do` keyword. //! - The `<-` syntactic sugar binds its left hand side to the monadic right hand side //! by _entering_ the right side via a closure. //! - Like almost any statement in Rust, you must end your statement with a semicolon (`;`). //! - The last line must be absent of `;` or contains the `return` keyword. //! - You can use `return` nowhere but on the last line. //! - A line containing a single expression with a semicolon is a valid statement and has the same effect as `_ <- expr`. //! //! ## How do I make my monad works with `m!`? //! //! Because monads are higher-kinded types, it is not possible to define the monadic do-notation in a fully type-system //! elegant way. However, this crate is based on the rebindable concept in Haskell (i.e. you can change what the `>>=` //! operator’s types are), so `m!` has one type-system requirement and one syntactic requirement. //! //! First, you have to implement one trait: [`Lift`], which allows to _lift_ a value `A` into a _monadic structure of //! `A`_. For instance, lifting a `A` into the `Option` monad yields an `Option<A>`. //! //! Then, you have to provide an `and_then` method, which is akin to Haskell’s `>>=` operator. The choice of using //! `and_then` and not a proper name like `flat_map` or `bind` is due to the current state of the standard-library — //! monads like `Option` and `Result<_, E>` don’t have `flat_map` defined on them but have `and_then`. The type signature //! is not enforced, but: //! //! - `and_then` must be a binary function taking a type `A`, a closure `A -> Monad<B>` and returns `Monad<B>`, where //! `Monad` is the monad you are adding `and_then` for. For instance, if you are implementing it for `Option`, //! `and_then` takes an `A`, a closure `A -> Option<B>` and returns an `Option<B>`. //! - `and_then` must move its first argument, which has to be `self`. The type of `Self` is not enforced. //! - `and_then`’s closure must take `A` with a `FnOnce` closure. //! //! ## Meaning of the `<-` operator //! //! The `<-` syntactic sugar is not strictly speaking an operator: it’s not valid vanilla Rust. Instead, it’s a trick //! defined in the `m!` allowing to use both [`Lift::lift`] and `and_then`. When you look at code inside a do-notation //! block, every monadic statements (separated with `;` in this crate) can be imagined as a new level of nesting inside //! a closure — the one passed to `and_then`, indeed. //! //! ## First example: fallible code //! //! One of the first monadic application that people learn is the _fallible_ effect — `Maybe` in Haskell. //! In `Rust`, it’s `Option`. `Option` is an interesting monad as it allows you to fail early. //! //! ```rust //! use do_notation::m; //! //! let r = m! { //! x <- Some("Hello, world!"); //! y <- Some(3); //! Some(x.len() * y) //! }; //! //! assert_eq!(r, Some(39)); //! ``` //! //! The `binding <- expr` syntax unwraps the right part and binds it to `binding`, making it available to //! next calls — remember, nested closures. The final line re-enters the structure (here, `Option`) explicitly. //! //! Note that it is possible to re-enter the structure without having to specify how / knowing the structure //! (with `Option`, you re-enter with `Some`). You can use the `return` keyword, that will automatically lift the //! value into the right structure: //! //! ```rust //! use do_notation::m; //! //! let r = m! { //! x <- Some(1); //! y <- Some(2); //! z <- Some(3); //! return [x, y, z]; //! }; //! //! assert_eq!(r, Some([1, 2, 3])); //! ``` #[macro_export] macro_rules! m { // return (return $r:expr ;) => { $crate::Lift::lift($r) }; // const-bind (_ <- $x:expr ; $($r:tt)*) => { $x.and_then(|_| { m!($($r)*) }) }; // bind ($binding:ident <- $x:expr ; $($r:tt)*) => { $x.and_then(|$binding| { m!($($r)*) }) }; // const-bind ($e:expr ; $($a:tt)*) => { $e.and_then(|_| m!($($a)*)) }; // pure ($a:expr) => { $a } } /// Lift a value inside a monad. pub trait Lift<A> { /// Lift a value into a default structure. fn lift(a: A) -> Self; } impl<A> Lift<A> for Option<A> { fn lift(a: A) -> Self { Some(a) } } impl<A, E> Lift<A> for Result<A, E> { fn lift(a: A) -> Self { Ok(a) } } #[cfg(test)] mod tests { use super::*; #[test] fn option() { let r: Option<i32> = m! { v <- Some(3); Some(v) }; assert_eq!(r, Some(3)); let r: Option<i32> = m! { v <- r; x <- Some(10); Some(v * x) }; assert_eq!(r, Some(30)); let n: Option<i32> = None; let r: Option<i32> = m! { v <- Some(314); x <- n; Some(v * x) }; assert_eq!(r, None); let r = m! { _ <- Some("a"); b <- Some("b"); _ <- Option::<&str>::None; Some(b) }; assert_eq!(r, None); let r = m! { _ <- Some("a"); return "b"; }; assert_eq!(r, Some("b")); } #[test] fn result() { let r: Result<i32, &str> = m! { v <- Ok(3); Ok(v) }; assert_eq!(r, Ok(3)); let r: Result<i32, &str> = m! { v <- r; x <- Ok(10); Ok(v * x) }; assert_eq!(r, Ok(30)); let n: Result<i32, &str> = Err("error"); let r: Result<i32, &str> = m! { v <- Ok(314); x <- n; Ok(v * x) }; assert_eq!(r, Err("error")); let r = m! { _ <- Result::<&str, &str>::Ok("a"); b <- Ok("b"); _ <- Result::<&str, &str>::Err("nope"); Ok(b) }; assert_eq!(r, Err("nope")); fn guard<E>(cond: bool, err: E) -> Result<(), E> { if cond { Ok(()) } else { Err(err) } } let r = m! { x <- Ok(true); _ <- guard(1 == 2, "meh"); Ok(x) }; assert_eq!(r, Err("meh")); } #[test] fn instruction_counter() { struct IC<A> { count: usize, value: A, } impl<A> IC<A> { fn new(value: A) -> Self { IC { count: 1, value } } fn value(&self) -> &A { &self.value } fn count(&self) -> usize { self.count } fn and_then<B>(self, f: impl FnOnce(A) -> IC<B>) -> IC<B> { let r = f(self.value); IC { count: self.count + r.count, value: r.value, } } } impl<A> Lift<A> for IC<A> { fn lift(value: A) -> Self { Self::new(value) } } let ic = m! { a <- IC::new(10); b <- IC::new(2); IC::new(a + b) }; assert_eq!(ic.value(), &12); assert_eq!(ic.count(), 3); let ic = m! { _ <- IC::new("a"); return [1, 2, 3]; }; assert_eq!(ic.value(), &[1, 2, 3]); assert_eq!(ic.count(), 2); } }