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//! # Smart accessors //! //! Let's begin with a few words on naming. //! //! What is commonly named “smart __pointer__” is usually associated //! with trivial access (dereference) semantics and nontrivial clone/drop semantics. //! //! Smart __accessors__ provided by this crate also serve the purpose of //! accessing some data, but in a different way: they have trivial drop semantics //! and nontrivial access semantics. //! //! ## TLDR //! //! If you do not want to read a long text, just proceed to the //! [essential part](#cargo-features) of the documentation. //! //! Also there is the [version migration guide](#version-migration-guide). //! //! //! ## High level overview //! //! The goal of this crate is threefold: //! //! * to offer one possible solution to the //! [problem](https://rust-lang.github.io/rfcs/2094-nll.html#problem-case-3-conditional-control-flow-across-functions) that //! the current (rustc 1.44) borrowchecker doesn't understand //! functions with multiple exit points //! ([Polonius](https://github.com/rust-lang/polonius) //! doesn't have this problem but it is not stable yet) //! * to provide a way to do bidirectional programming (when updating //! some view of data updates the data viewed to match the updated view) //! * last but not least, to reduce amount of callback hell //! while programming in continuation-passing-style //! //! If you are aqcuainted with optics in functional languages you can //! think of this crate as a minimalistic “lens” (more precisely, //! affine traversal) library using an opionated imperative approach. //! //! ### Note //! //! As a side-effect of the library design one can use a “build” //! pattern with standard `&mut` references (see below). //! //! //! ## Usage examples //! //! This crate already implements [accessors](stdlib_impls/) for stdlib collections: //! //! ``` //! use smart_access::Cps; //! //! let mut foo = vec![vec![1,2,3], vec![4,5,6]]; //! //! let bar = foo.at(0).at(1).replace(7); //! assert!(foo == vec![vec![1,7,3], vec![4,5,6]]); //! assert!(bar == Some(2)); //! //! let baz = foo.at(2).at(1).replace(8); //! assert!(foo == vec![vec![1,7,3], vec![4,5,6]]); //! assert!(baz == None); // None is returned if path doesn't make sense //! //! // Any mutable reference can be used as a "data location": //! assert!(foo[0][0].replace(9) == Some(1)); //! assert!(foo == vec![vec![9,7,3], vec![4,5,6]]); //! ``` //! //! A somewhat more interesting example: //! //! ``` //! # use smart_access::Cps; //! let mut foo = vec![1,2,3,4,5,6]; //! //! let bar = foo.at(1..=3).replace(vec![7,8]); //! assert!(foo == vec![1,7,8,5,6]); //! assert!(bar == Some(vec![2,3,4])); //! ``` //! //! An arbitrary mutating operation can be used instead of replacement: //! //! ``` //! # use smart_access::Cps; //! let mut foo = vec![1,2,3,4,5,6]; //! //! let bar = foo.at(1..4).access(|v| { *v = vec![v.iter().sum()]; "baz" }); //! assert!(foo == vec![1,9,5,6]); //! assert!(bar == Some("baz")); //! //! // And with mutable references you get a sort of the "build" part of the Builder pattern //! foo[0].access(|x| { /* do something with the element */ }); //! ``` //! //! //! ## Usage guide //! //! To add a smart accessor to your own datatype `Data` you need to: //! //! * choose some type `Index` //! * add trait [`At<Index>`](trait.At.html) to the type `Data` //! * implement [`access_at`](trait.At.html#tymethod.access_at) method //! * at the usage site write `use smart_access::Cps;` //! * PROFIT! //! //! //! ## Motivation (part I: lifetimes) //! //! Suppose you have `HashMap` but without “Entry API” //! (Entry API is an implementation feature: not every datastructure //! in the wild provides any analogue). //! //! Suppose also that you want to implement something akin to //! `|m, k, v| m.entry(k).or_insert(v)`. //! //! You could write //! //! ``` compile_fail //! # use std::collections::HashMap; //! // for simplicity we use usize keys in the examples below //! fn or_insert<V>(hm: &mut HashMap<usize,V>, k: usize, v: V) -> &mut V { //! if let Some(v) = hm.get_mut(&k) { //! return v; //! // this is the first exit point but the borrow checker //! // doesn't distinguish between it and the second exit point //! } //! //! hm.insert(k, v); // Oops: hm is already borrowed! //! // (It _MUST_ be borrowed until the exit point) //! //! hm.get_mut(&k).unwrap() //! // the second exit point //! } //! ``` //! //! but it would not compile because of limitations of the borrow checker. //! //! It seems there is no way to write such a function without //! additional queries to the `HashMap` and without //! resorting to reference-pointer-reference conversions or //! other `unsafe` techniques. //! //! This crate provides a not-so-clumsy workaround: //! //! ``` //! use std::collections::HashMap; //! use smart_access::{At, Cps}; //! //! struct Ensure<K,V> { key: K, value: V } //! //! impl<V> At<Ensure<usize, V>> for HashMap<usize, V> //! { //! type View = V; //! //! fn access_at<R, F>(&mut self, kv: Ensure<usize, V>, f: F) -> Option<R> where //! F: FnOnce(&mut V) -> R //! { //! if let Some(v) = self.get_mut(&kv.key) { //! return Some(f(v)); //! // We use the so called CPS-transformation: we wrap each //! // return site with a call to the provided function. //! } //! //! self.insert(kv.key, kv.value); //! Some(f(self.get_mut(&kv.key).unwrap())) //! } //! } //! //! // now you can write or_insert (note the return type!): //! fn or_insert<'a, V>(hm: &'a mut HashMap<usize,V>, k: usize, v: V) -> impl Cps<View=V> + 'a { //! hm.at(Ensure{ key: k, value: v }) //! } //! ``` //! //! There are some peculiarities though: //! //! * `&mut V` is _eager_: all code which is needed to obtain a reference //! to the value is executed at the site of access //! * `impl Cps<View=V>` is _lazy_: access is a zero-cost operation and all //! the machinery needed to reach the value is run at the site of modification //! * `&'a mut V` can be reborrowed, i.e. cloned for some subperiod of `'a`, //! making it possible to modify the value referenced more than once //! * `impl Cps<View=V>` can be used only once but has [batching](struct.CpsBatch.html). //! It comes in two flavors: _compile-time batching_ //! which can't be used across any control flow and _runtime batching_ which //! can't be used in `no_std` contexts //! //! ### Note //! //! The forementioned accessor `Ensure { key: K, value: V }` is defined //! in [`stdlib_impls`](stdlib_impls/) simply as a pair `(K,V)` so //! for example you can write //! //! ``` //! # use smart_access::Cps; //! # let mut map = hashbrown::HashMap::<String,String>::new(); //! map.at( ("foo".to_string(), "bar".to_string()) ).touch(); //! ``` //! //! instead of //! //! ``` //! # let mut map = hashbrown::HashMap::<String,String>::new(); //! map.entry("foo".to_string()).or_insert("bar".to_string()); //! ``` //! //! //! ## Motivation (part II: bidirectional programming) //! //! We give a simple illustration: a toy example of a bidirectional vector parser. //! //! Not only can it parse a vector but also can print it back (note //! that two bidirectional parsers can be combined into a bidirectional //! translator from one textual representation to another). //! //! A combinator library greatly facilitating writing such parsers //! can be implemented but it is not a (current-time) goal of this crate. //! //! ### Note //! //! Some function definitions in the following code are hidden. To see them look //! at the full [module source](../src/smart_access/lib.rs.html). //! //! ``` //! // A little showcase: //! assert!(vector_parser().bi_left((Some(vec![1,2,3]),"".into())) == "[1,2,3]".to_string()); //! assert!(vector_parser().bi_right(&mut "[1,2,3] foo".into()).0 == Some(vec![1,2,3])); //! assert!(vector_parser().bi_right(&mut "[1,2,3,]bar".into()).0 == Some(vec![1,2,3])); //! assert!(vector_parser().bi_right(&mut "[,]".into()).0 == None); //! assert!(vector_parser().bi_right(&mut "[]".into()).0 == Some(vec![])); //! assert!(vector_parser().bi_right(&mut "]1,2,3[".into()).0 == None); //! //! // The code: //! use smart_access::{At, Cps}; //! //! // a minimal set of parser combinators //! #[derive(Clone)] struct _Number; //! #[derive(Clone)] struct _Char(char); //! #[derive(Clone)] struct _Many<T>(T); //! #[derive(Clone)] struct _Optional<T>(T); //! #[derive(Clone)] struct _Cons<Car,Cdr>(Car,Cdr); //! #[derive(Clone)] struct _Iso<Parser,F,G>(Parser,F,G); //! //! fn vector_parser() -> impl Bidirectional<String, Parse<Vec<usize>>> { //! let grammar = //! _Cons(_Char('['), //! _Cons(_Many(_Cons(_Number, _Char(','))), //! _Cons(_Optional(_Number), //! _Char(']')))); //! //! let from_grammar = |(_bl, (xs, (ox, _br))): (_, (Vec<_>, (Option<_>, _)))| //! { //! xs.into_iter().map(|(x, _comma)| x).chain(ox.into_iter()).collect() //! }; //! //! let to_grammar = |mut vec: Vec<_>| { //! let last = vec.pop(); //! //! ('[', (vec.into_iter().map(|x| (x, ',')).collect(), (last, ']'))) //! }; //! //! _Iso(grammar, from_grammar, to_grammar) //! } //! //! trait Bidirectional<A,B> { //! fn bi_left(self, b: B) -> A; //! fn bi_right(self, a: &mut A) -> B; //! } //! //! // DO NOT USE IN PRODUCTION: efficient parsing is incompatible //! // with using copies of tails of the parsed string //! type Parse<T> = (Option<T>, String); //! //! // a very simplistic blanket implementation //! impl<A,B,I> Bidirectional<A,B> for I where //! A: At<I, View=B> + Default, //! B: Clone //! { //! fn bi_left(self, b: B) -> A { //! let mut a = A::default(); //! //! a.at(self).access(|x| { *x = b; }); //! //! a //! } //! //! fn bi_right(self, a: &mut A) -> B { //! a.at(self).access(|b| b.clone()).unwrap() //! } //! } //! //! impl At<_Number> for String { //! type View = Parse<usize>; //! //! # fn access_at<R,F>(&mut self, _: _Number, f: F) -> Option<R> where //! # F: FnOnce(&mut Parse<usize>) -> R //! # { //! # let mut digits = String::new(); //! # //! # let mut it = self.chars(); //! # let mut maybe_c = None; //! # for c in &mut it { //! # if c >= '0' && c <= '9' { digits.push(c); } //! # else { maybe_c = Some(c); break; } //! # } //! # //! # let rest = maybe_c.into_iter().chain(it).collect::<String>(); //! # let mut arg = match digits.parse() { //! # Err(_) => (None, self.clone()), //! # Ok(number) => (Some(number), rest), //! # }; //! # //! # let result = f(&mut arg); //! # //! # let (maybe_number, rest) = arg; //! # match maybe_number { //! # Some(number) => { *self = number.to_string() + &rest; } //! # None => { *self = rest; } //! # } //! # //! # Some(result) //! # } //! // access_at is hidden //! } //! //! impl At<_Char> for String { //! type View = Parse<char>; //! //! # fn access_at<R,F>(&mut self, i: _Char, f: F) -> Option<R> where //! # F: FnOnce(&mut Parse<char>) -> R //! # { //! # let mut it = self.chars(); //! # //! # let mut arg = match it.next() { //! # None => { (None, self.clone()) } //! # Some(c) => { //! # if c != i.0 { (None, self.clone()) } //! # else { (Some(c), it.collect::<String>()) } //! # } //! # }; //! # //! # let result = f(&mut arg); //! # //! # let (maybe_c, rest) = arg; //! # match maybe_c { //! # Some(c) => { *self = c.to_string() + &rest; } //! # None => { *self = rest; } //! # } //! # //! # Some(result) //! # } //! // access_at is hidden //! } //! //! impl<V, Parser> At<_Many<Parser>> for String where //! String: At<Parser, View=Parse<V>>, //! Parser: Bidirectional<String, Parse<V>> + Clone, //! { //! type View = Parse<Vec<V>>; //! //! # fn access_at<R,F>(&mut self, i: _Many<Parser>, f: F) -> Option<R> where //! # F: FnOnce(&mut Self::View) -> R //! # { //! # let parser = &i.0; //! # //! # let mut vec = Vec::<V>::new(); //! # let mut current_string = self.clone(); //! # //! # loop { //! # match parser.clone().bi_right(&mut current_string) { //! # (Some(v),s) => { //! # vec.push(v); //! # current_string = s; //! # } //! # //! # (None,_) => { break; } //! # } //! # } //! # //! # let mut arg = (Some(vec), current_string); //! # let result = f(&mut arg); //! # //! # let (maybe_vec, rest) = arg; //! # match maybe_vec { //! # None => { *self = rest; } //! # Some(vec) => { //! # *self = vec.into_iter() //! # .map(|x| parser.clone().bi_left((Some(x),"".into()))) //! # .collect::<String>() + &rest; //! # } //! # } //! # //! # Some(result) //! # } //! // access_at is hidden //! } //! //! impl<V, Parser> At<_Optional<Parser>> for String where //! String: At<Parser, View=Parse<V>>, //! Parser: Bidirectional<String, Parse<V>> + Clone, //! { //! type View = Parse<Option<V>>; //! //! # fn access_at<R,F>(&mut self, i: _Optional<Parser>, f: F) -> Option<R> where //! # F: FnOnce(&mut Self::View) -> R //! # { //! # let parser = i.0; //! # //! # let mut arg = match parser.clone().bi_right(self) { //! # (maybe_value, s) => (Some(maybe_value), s), //! # }; //! # //! # let result = f(&mut arg); //! # //! # let (maybe_value, rest) = arg; //! # match maybe_value { //! # None => { *self = rest; } //! # Some(maybe_value) => { //! # *self = parser.bi_left((maybe_value,"".into())) + &rest; //! # } //! # } //! # //! # Some(result) //! # } //! // access_at is hidden //! } //! //! impl<V1, V2, P1, P2> At<_Cons<P1, P2>> for String where //! String: At<P1, View=Parse<V1>>, //! String: At<P2, View=Parse<V2>>, //! P1: Bidirectional<String, Parse<V1>> + Clone, //! P2: Bidirectional<String, Parse<V2>> + Clone, //! { //! type View = Parse<(V1,V2)>; //! //! # fn access_at<R,F>(&mut self, i: _Cons<P1, P2>, f: F) -> Option<R> where //! # F: FnOnce(&mut Self::View) -> R //! # { //! # let _Cons(p1, p2) = i; //! # //! # let (maybe_v1, mut s1) = p1.clone().bi_right(self); //! # let (maybe_v2, s2) = p2.clone().bi_right(&mut s1); //! # //! # let mut arg = match (maybe_v1, maybe_v2) { //! # (Some(v1), Some(v2)) => (Some( (v1, v2) ), s2), //! # _ => (None, self.clone()) //! # }; //! # //! # let result = f(&mut arg); //! # //! # let (maybe_values, rest) = arg; //! # match maybe_values { //! # None => { *self = rest; } //! # Some( (v1, v2) ) => { //! # *self = vec![ //! # p1.bi_left((Some(v1), "".into())), //! # p2.bi_left((Some(v2), "".into())), //! # rest //! # ].into_iter().collect(); //! # } //! # } //! # //! # Some(result) //! # } //! // access_at is hidden //! } //! //! impl<Parser, FromParser, ToParser, T, V> //! At<_Iso<Parser, FromParser, ToParser>> for String where //! String: At<Parser, View=Parse<T>>, //! Parser: Bidirectional<String, Parse<T>> + Clone, //! T: Clone, //! FromParser: FnOnce(T) -> V, //! ToParser: FnOnce(V) -> T, //! { //! type View = Parse<V>; //! //! # fn access_at<R,F>(&mut self, i: _Iso<Parser, FromParser, ToParser>, f: F) //! # -> Option<R> where //! # F: FnOnce(&mut Self::View) -> R //! # { //! # let _Iso(parser, from_parser, to_parser) = i; //! # //! # let (maybe_t, rest) = parser.clone().bi_right(self); //! # //! # let mut arg = (maybe_t.map(|t| from_parser(t)), rest); //! # let result = f(&mut arg); //! # //! # let (maybe_v, rest) = arg; //! # match maybe_v { //! # None => { *self = rest; } //! # Some(v) => { //! # *self = parser.bi_left((Some(to_parser(v)),"".to_string())) + &rest; //! # } //! # } //! # //! # Some(result) //! # } //! // access_at is hidden //! } //! ``` //! //! //! ## Connection to functional programming //! //! Rust type `fn(&mut V)` roughly corresponds to Haskell type `v -> v`. //! //! Thus Rust [`access_at`](trait.At.html#tymethod.access_at) type //! could be written in Haskell (after some argument-swapping) as //! //! ``` haskell //! accessAt :: ix -> (v -> (v,r)) -> t -> (t, Maybe r) //! ``` //! //! Suppose that `access_at` always returns `Some(..)`. In such a case //! the Haskell type of `access_at` can be simplified to //! //! ``` haskell //! accessAt :: ix -> (v -> (v,r)) -> t -> (t,r) //! ``` //! //! Its type is isomorphic to any of the following //! //! ``` haskell //! ix -> t -> (v -> (v,r)) -> (t,r) -- by arg-swapping //! ix -> t -> (v->v, v->r) -> (t,r) -- by universal property of products //! ix -> t -> (v->v) -> (v->r) -> (t,r) -- by currying //! ``` //! //! Recall that a lens is uniquely defined by a getter and a setter: //! //! ``` haskell //! type Lens t v = (t -> v, t -> v -> t) //! ``` //! //! This type is isomorphic to //! //! ``` haskell //! type Lens t v = t -> (v, v -> t) //! ``` //! //! Notice that the types `(v, v->t)` and `forall r. (v->v) -> (v->r) -> (t,r)` //! are rather similiar. Define //! //! ``` haskell //! right :: (v, v->t) -> (v->v) -> (v->r) -> (t,r) //! right (v, v_t) v_v v_r = (v_t (v_v v), v_r v) //! //! left :: (forall r. (v->v) -> (v->r) -> (t,r)) -> (v, v->t) //! left f = (snd (f id id ), -- getter //! \v -> fst (f (\_ -> v) (\_ -> ()))) -- setter //! ``` //! //! Now we prove `(left . right) ~ id`: //! //! ``` haskell //! left (right (v, v_t)) = (v, \x -> v_t x) ~ (v, v_t) //! ``` //! //! I.e. `right` is an injection: every value `lens :: Lens t v` can be //! presented as `left (accessAt ix)`: it suffices to define //! //! ``` haskell //! accessAt ix = right lens -- modulo aforementioned type-fu //! ``` //! //! In fact the full type (with `Maybe`) //! //! ``` haskell //! accessAt ix :: (v -> (v,r)) -> t -> (t, Maybe r) //! ``` //! //! can house any lens, prism or affine traversal. //! //! ## Version migration guide //! //! ### From 0.6 to 0.7 //! //! #### Difference #1 //! //! Now the crate is fully independent of `std`. There is one thing however: //! `HashMap` and `HashSet` are not yet in the `alloc::collections`. //! //! There are two options: //! //! * use the optional `hashbrown` dependency //! * link to `std` by enabling the `std_hashmap` feature //! //! //! #### Difference #2 //! //! The first iteration of a general traversal (not affine, i.e. supporting //! more than one location) API is available. Obviously, it's fully //! backward-compatible. //! //! //! ### From 0.5 to 0.6 //! //! #### Difference #1 //! //! The `std_collections` feature is now named `collections`. //! //! The `std` feature is deprecated. In the next version it will very likely be //! replaced by the `alloc` feature, responsible for linking to the `alloc` crate. //! //! #### Difference #2 //! //! There is a new feature [`iter_mut`](./iter_mut/). //! It depends on the external crate //! [`multiref`](https://crates.io/crates/multiref/). That crate //! doesn't use any complex type-level programming, thus making //! the increase in the compilation time insignificant. //! //! But if you do not tolerate any dependencies or unsafe code, //! you can opt it out, losing some expressiveness. //! //! //! ### From 0.4 to 0.5 //! //! #### Difference #1 //! //! The [`AT`](struct.AT.html) type changed its representation. //! //! Now it has simpler and more flat structure: //! //! `AT<CPS, (..(((), I1), I2) .. In)>` //! //! instead of //! //! `AT<..AT<AT<CPS, I1>, I2> .. In>` //! //! Unfortunately, the new structure doesn't play well with type inference //! but this issue has been circumvented by separating the `Cps` implementation //! into a helper trait (this trait isn't exposed to the public API). //! //! Because the `AT` type isn't to be used explicitly, usually there is //! no need to change any code. //! //! Nevertheless there may exist some code which has compiled on 0.4 and does not //! compile on 0.5. //! //! #### Difference #2 //! //! Relevant only to the `detach` feature. //! //! Now the [`detach`](struct.At.html#method.detach) method returns not //! only the detached part but also the left part (an accessor with //! the rest of the path attached). //! //! A new method [`cut`](trait.Cps.html#method.cut) is provided. //! It allows one to mark the place from which the detach starts. //! //! #### Difference #3 //! //! Also about `detach`. //! //! The [`Attach`](trait.Attach.html) trait changed its parameter to `View` //! instead of `CPS`. //! //! Usually it's sufficient to change `CPS` to `CPS::View` in generic code. //! //! Interestingly, there are some cases when //! //! ``` //! # use smart_access::*; #[cfg(feature="detach")] //! fn foo<CPS: Cps, Path: Attach<CPS::View>> // ... //! # (){} //! ``` //! //! is not equivalent to //! //! ``` //! # use smart_access::*; #[cfg(feature="detach")] //! fn foo<CPS: Cps<View=V>, Path: Attach<V>, V: ?Sized> // ... //! # (){} //! ``` //! //! For example, `impl Attach<V>` works after being returned from a function //! but `impl Attach<CPS::View>` doesn't (it seems to be connected with //! the fact that Rust currenlty doesn't “elaborate” (i.e. reduce) //! bounds on the associated types). //! //! ## Cargo features //! //! Currently there are following features: //! //! * `alloc`: Links to `alloc`. //! * `collections`: Provides [accessors for some collections](./collections/). //! __Implies `alloc`.__ //! * `hashbrown`: Accessors for `HashMap` and `HashSet` from the //! [`hashbrown`](https://crates.io/crates/hashbrown) crate. //! __Pulls the `hashbrown` crate, implies `alloc`.__ //! * `std_hashmap`: Accessors for `HashMap` and `HashSet` from `std`. __Warning: links to `std`.__ //! * `batch_rt`: Provides runtime [batching](struct.CpsBatch.html). //! __Implies `alloc`.__ //! * `batch_ct`: Provides compile-time [batching](struct.CpsBatch.html). //! * `batch`: An alias for `batch_rt` and `batch_ct` enabled simultaneously. //! * `detach`: Makes [`AT`](struct.AT.html)-paths [detachable](struct.AT.html#method.detach). //! * `iter_mut`: [Accessors for iterators](./iter_mut/). //! __Pulls the [`multiref`](https://crates.io/crates/multiref) crate, implies `alloc`.__ //! * `traversal`: Bidirectional iterators in continuation passing style. //! //! All features except `std_hashmap` are enabled by default. #![no_std] #[cfg(feature="alloc")] extern crate alloc; mod at; pub mod core_impls; #[cfg(feature="collections")] pub mod collections; pub use at::{At, AT, Cps}; #[cfg(any(feature="batch_rt", feature="batch_ct"))] mod batch; #[cfg(any(feature="batch_rt", feature="batch_ct"))] pub use batch::{ CpsBatch, Batch }; #[cfg(feature="batch_ct")] pub use batch::{ BatchCt }; #[cfg(feature="batch_rt")] pub use batch::{ BatchRt }; #[cfg(feature="detach")] pub use at::{ Attach, detached_at, DetachedPath }; #[cfg(feature="iter_mut")] pub mod iter_mut; #[cfg(feature="traversal")] pub use at::traversal; mod macros;