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//! The bidule FRP crate. //! //! This crate provides a few simple primitives to write FRP-driven programs. Everything revolves //! around the concept of a `Stream`. //! //! # Streams //! //! A `Stream` is a *stream of typed signals*. A stream of signals will get a *signal* as input //! and will broadcast it downwards. You can compose streams with each other with very simple //! combinators, such as `map`, `filter`, `filter_map`, `zip`, `unzip`, `merge`, `fold`, `sink`, //! etc. //! //! ## Creating streams and send signals //! //! Streams are typed. You can use type inference or give them an explicit type: //! //! ``` //! use bidule::Stream; //! //! let my_stream: Stream<i32> = Stream::new(); //! ``` //! //! That’s all you need to create a stream. A stream represent a value that will be flowing in *at //! some time*. //! //! > Even though it’s not strictly the same thing, you can see a similitude with //! > [futures](https://crates.io/crates/futures). //! //! When you’re ready to send signals, just call the `send` function: //! //! ``` //! use bidule::Stream; //! //! let my_stream: Stream<i32> = Stream::new(); //! //! my_stream.send(&1); //! my_stream.send(&2); //! my_stream.send(&3); //! ``` //! //! ## Subscriptions //! //! A single stream like that one won’t do much – actually, it’ll do nothing. The first thing we //! might want to do is to subscribe a closure to do something when a signal is emitted. This is //! done with the `subscribe` function. //! //! ``` //! use bidule::Stream; //! //! let my_stream: Stream<i32> = Stream::new(); //! //! my_stream.subscribe(|sig| { //! // print the signal on stdout each time it’s flowing in //! println!("signal: {:?}", sig); //! }); //! //! my_stream.send(&1); //! my_stream.send(&2); //! my_stream.send(&3); //! ``` //! //! ## FRP basics //! //! However, FRP is not about callbacks. It’s actually the opposite, to be honest. We try to reduce //! the use of callbacks as much as possible. FRP solves this by inversing the way you must work: //! instead of subscribing callbacks to react to something, you transform that something to create //! new values or objects. This is akin to the kind of transformations you do with `Future`. //! //! Let’s get our feet wet: let’s create a new stream that will only emit signals for even values: //! //! ``` //! use bidule::Stream; //! //! let int_stream: Stream<i32> = Stream::new(); //! let even_stream = int_stream.filter(|x| x % 2 == 0); //! ``` //! //! `even_stream` has type `Stream<i32>` and will only emit signals when the input signal is `even`. //! //! Let’s try something more complicated: on those signals, if the value is less or equal to 10, //! output `"Hello, world!"`; otherwise, output `"See you!"`. //! //! ``` //! use bidule::Stream; //! //! let int_stream: Stream<i32> = Stream::new(); //! let even_stream = int_stream.filter(|x| x % 2 == 0); //! let str_stream = even_stream.map(|x| if *x <= 10 { "Hello, world!" } else { "See you!" }); //! ``` //! //! This is really easy; no trap involved. //! //! Ok, let’s try something else. Some kind of a *Hello world* for FRP. //! //! ``` //! use bidule::Stream; //! //! enum Button { //! Pressed, //! Released //! } //! //! fn unbuttonify(button: &Button, v: i32) -> Option<i32> { //! match *button { //! Button::Released => Some(v), //! _ => None //! } //! } //! //! let minus = Stream::new(); //! let plus = Stream::new(); //! let counter = //! minus.filter_map(|b| unbuttonify(b, -1)) //! .merge(&plus.filter_map(|b| unbuttonify(b, 1))) //! .fold(0, |a, x| a + x); //! ``` //! //! In this snippet, we have two buttons: `minus` and `plus`. If we hit the `minus` button, we want //! a counter to be decremented and if we hit the `plus` button, the counter must increment. //! //! FRP solves that problem by expressing `counter` in terms of both `minus` and `plus`. The first //! thing we do is to map a number on the stream that broadcasts button signals. Whenever that //! signal is a `Button::Released`, we return a given number. For `minus`, we return `-1` and for //! `plus`, we return `1` – or `+1`, it’s the same thing. That gives us two new streams. Let’s see //! the types to have a deeper understanding: //! //! - `minus: Stream<Button>` //! - `plus: Stream<Button>` //! - `minus.filter_map(|b| unbuttonify(b, -1)): Stream<i32>` //! - `plus.filter_map(|b| unbuttonify(b, 1)): Stream<i32>` //! //! The `merge` method is very simple: it takes two streams that emit the same type of signals and //! merges them into a single stream that will broadcasts both the signals: //! //! - `minus.filter_map(|b| unbuttonify(b, -1)).merge(plus.filter_map(|b| unbuttonify(b, 1)): Stream<i32>): Stream<i32>` //! //! The next and final step is to `fold` those `i32` into the final value of the counter by applying //! successive additions. This is done with the `fold` method, that takes the initial value – in the //! case of a counter, it’s `0` – and the function to accumulate, with the accumulator as first //! argument and the iterated value as second argument. //! //! The resulting stream, which type is `Stream<i32>`, will then contain the value of the counter. //! You can test it by sending `Button` signals on both `minus` and `plus`: the resulting signal //! in `counter` will be correctly decrementing or incrementing. //! //! There exist several more, interesting combinators to work with your streams. For instance, if //! you don’t want to map a function over two streams to make them compatible with each other – they //! have different types, you can still perform some kind of a merge. That operation is called a //! `zip` and the resulting stream will yield either the value from the first – left – stream or the //! value of the other – right – as soon as a signal is emitted. Its dual method is called `unzip` //! and will split a stream apart into two streams if it’s a zipped stream. See `Either` for further //! details. //! //! ## Sinking //! //! *Sinking* is the action to consume the signals of a stream and collect them. The current //! implementation uses non-blocking buffering when sending signals, and reading is up to you: the //! stream will collect the output signals in a buffer you can read via the //! [Iterator](https://doc.rust-lang.org/std/iter/trait.Iterator.html) trait. For instance, //! non-blocking reads: //! //! ``` //! use bidule::Stream; //! //! enum Button { //! Pressed, //! Released //! } //! //! fn unbuttonify(button: &Button, v: i32) -> Option<i32> { //! match *button { //! Button::Released => Some(v), //! _ => None //! } //! } //! //! let minus = Stream::new(); //! let plus = Stream::new(); //! let counter = //! minus.filter_map(|b| unbuttonify(b, -1)) //! .merge(&plus.filter_map(|b| unbuttonify(b, 1))) //! .fold(0, |a, x| a + x); //! //! let rx = counter.sink(); //! //! // do something with minus and plus //! // … //! //! for v in rx.try_iter() { //! println!("read a new value of the counter: {}", v); //! } //! ``` use std::cell::RefCell; use std::rc::{Rc, Weak}; use std::sync::mpsc::{Receiver, channel}; type Subscribers<'a, Sig> = RefCell<Vec<Box<FnMut(&Sig) + 'a>>>; enum SubscribersRef<'a, Sig> { Own(Rc<Subscribers<'a, Sig>>), Weak(Weak<Subscribers<'a, Sig>>) } /// Either one or another type. /// /// This type is especially useful for zipping and unzipping streams. If a stream has a type like /// `Stream<Either<A, B>>`, it means you can unzip it and get two streams: `Stream<A>` and /// `Stream<B>`. #[derive(Clone, Debug, Eq, PartialEq)] pub enum Either<A, B> { Left(A), Right(B) } /// A stream of signals. /// /// A stream represents a composable signal producer. When you decide to send a signal down a /// stream, any other streams composed with that first stream will also receive the signal. This /// enables to construct more interesting and complex streams by composing them. pub struct Stream<'a, Sig> { subscribers: SubscribersRef<'a, Sig> } impl<'a, Sig> Stream<'a, Sig> where Sig: 'a { /// Create a new stream. pub fn new() -> Self { let subscribers = SubscribersRef::Own(Rc::new(RefCell::new(Vec::new()))); Stream { subscribers } } /// Create a new, version of this stream by behaving the same way as the input reference (if it’s /// an owned pointer, it clones ownership; if it’s a weak pointer, it clone the weak pointer). fn new_same(&self) -> Self { let subscribers = match self.subscribers { SubscribersRef::Own(ref rc) => SubscribersRef::Own(rc.clone()), SubscribersRef::Weak(ref weak) => SubscribersRef::Weak(weak.clone()) }; Stream { subscribers } } /// Create new, non-owning version of this stream. fn new_weak(&self) -> Self { let subscribers = match self.subscribers { SubscribersRef::Own(ref rc) => SubscribersRef::Weak(Rc::downgrade(rc)), SubscribersRef::Weak(ref weak) => SubscribersRef::Weak(weak.clone()) }; Stream { subscribers } } /// Subscribe a new listener for this stream’s signals. /// /// This function enables to “observe” any signal flowing out of the stream. However, do not abuse /// this function, as its primary use is to build other combinators. pub fn subscribe<F>(&self, subscriber: F) where F: 'a + FnMut(&Sig) { match self.subscribers { SubscribersRef::Own(ref subscribers) => subscribers.borrow_mut().push(Box::new(subscriber)), SubscribersRef::Weak(ref weak) => { if let Some(subscribers) = weak.upgrade() { subscribers.borrow_mut().push(Box::new(subscriber)); } } } } /// Send a signal down the stream. pub fn send(&self, signal: &Sig) { match self.subscribers { SubscribersRef::Own(ref subscribers) => { for sub in subscribers.borrow_mut().iter_mut() { sub(signal); } } SubscribersRef::Weak(ref weak) => { if let Some(subscribers) = weak.upgrade() { for sub in subscribers.borrow_mut().iter_mut() { sub(signal); } } } } } /// Map any signals flowing out a stream. /// /// Please note that this function is total: you cannot ignore signals. Even if you map /// *uninteresting signals* to `None`, you’ll still compose signals for those. If are interested /// in filtering signals while mapping, have a look at the `filter_map` function. pub fn map<F, OutSig>( &self, f: F ) -> Stream<'a, OutSig> where F: 'a + Fn(&Sig) -> OutSig, OutSig: 'a { let mapped_stream = Stream::new(); let mapped_stream_ = mapped_stream.new_same(); self.subscribe(move |sig| { mapped_stream_.send(&f(sig)); }); mapped_stream } /// Filter and map signals flowing out a stream. /// /// If you’re not interested in a specific signal, you can emit `None`: no signal will be sent. pub fn filter_map<F, OutSig>( &self, f: F ) -> Stream<'a, OutSig> where F: 'a + Fn(&Sig) -> Option<OutSig>, OutSig: 'a { let mapped_stream = Stream::new(); let mapped_stream_ = mapped_stream.new_same(); self.subscribe(move |sig| { if let Some(ref mapped_sig) = f(sig) { mapped_stream_.send(mapped_sig); } }); mapped_stream } /// Filter the signals flowing out of a stream with a predicate. pub fn filter<F>(&self, pred: F) -> Self where F: 'a + Fn(&Sig) -> bool { let filtered = Stream::new(); let filtered_ = filtered.new_same(); self.subscribe(move |sig| { if pred(sig) { filtered_.send(sig); } }); filtered } /// Fold all signals flowing out of a stream into a stream of values. pub fn fold<F, A>( &self, value: A, f: F ) -> Stream<'a, A> where F: 'a + Fn(A, &Sig) -> A, A: 'a { let folded_stream = Stream::new(); let folded_stream_ = folded_stream.new_same(); let mut boxed = Some(value); self.subscribe(move |sig| { if let Some(value) = boxed.take() { let output = f(value, sig); folded_stream_.send(&output); boxed = Some(output); } }); folded_stream } /// Merge two streams into one. /// /// Merging streams enables you to perform later useful compositions, such as folding the merged /// results. pub fn merge(&self, rhs: &Self) -> Self { let merged = Stream::new(); let merged_self = merged.new_same(); let merged_rhs = merged.new_same(); self.subscribe(move |sig| { merged_self.send(sig); }); rhs.subscribe(move |sig| { merged_rhs.send(sig); }); merged } // FIXME: see whether we can do the same thing without Clone /// Zip two streams with each other. pub fn zip<SigRHS>( &self, rhs: &Stream<'a, SigRHS> ) -> Stream<'a, Either<Sig, SigRHS>> where Sig: Clone, SigRHS: 'a + Clone { let zipped = Stream::new(); let zipped_self = zipped.new_same(); let zipped_rhs = zipped.new_same(); self.subscribe(move |sig| { zipped_self.send(&Either::Left(sig.clone())); }); rhs.subscribe(move |sig| { zipped_rhs.send(&Either::Right(sig.clone())); }); zipped } /// Create a pair of entangled streams. /// /// If any of the streams sends a signal, the other one receives it. However, be careful: since /// the signals are defined in terms of each other, it’s quite easy to cause infinite loops if you /// don’t have a well-defined bottom to your recursion. This is why you’re expected to return /// `Option<_>` signals. pub fn entangled<F, G, GSig>( f: F, g: G ) -> (Self, Stream<'a, GSig>) where F: 'a + Fn(&Sig) -> Option<GSig>, G: 'a + Fn(&GSig) -> Option<Sig>, GSig: 'a { let fs = Stream::new(); let gs = Stream::new(); let fs_ = fs.new_weak(); let gs_ = gs.new_weak(); fs.subscribe(move |sig| { if let Some(sig_) = f(sig) { gs_.send(&sig_); } }); gs.subscribe(move |sig| { if let Some(sig_) = g(sig) { fs_.send(&sig_); } }); (fs, gs) } /// Sink a stream. pub fn sink(&self) -> Receiver<Sig> where Sig: Clone { let (sx, rx) = channel(); self.subscribe(move |sig| { let _ = sx.send(sig.clone()); }); rx } } impl<'a, SigA, SigB> Stream<'a, Either<SigA, SigB>> where SigA: 'static, SigB: 'static { /// Split a stream of zipped values into two streams. pub fn unzip(&self) -> (Stream<'a, SigA>, Stream<'a, SigB>) { let a = Stream::new(); let a_ = a.new_same(); let b = Stream::new(); let b_ = b.new_same(); self.subscribe(move |sig| { match *sig { Either::Left(ref l) => a_.send(l), Either::Right(ref r) => b_.send(r) } }); (a, b) } }