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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Functional Rust - An Exploration - partial::Conf 2018</title>
<link href="styles.css" rel="stylesheet">
</head>
<body>
<textarea id="source">
# .title[Functional Rust]
# .subtitle[An Exploration]
Lisa '[lislis](https://lislis.de/)' Passing
[partial::Conf](http://partialconf.com/) 2018
---
class: speaker-shot
background-image: url(img/coffee_mug.png)
## $ whoami
--
Developer at [Open Knowledge Foundation DE](https://okfn.de)
--
Game jammer, wannabe artist
--
FP through ClojureScript
--
Rustacean since early 2017
---
class: center, middle
## Rust!
---
## Rust is
--
- a systems programming language
--
- imperative, multi-paradigm
--
- not garbage collected and does not require manual memory management
--
⇒ ownership and borrowing could fill an entire talk on their own
--
- young, but popular ([1](https://insights.stackoverflow.com/survey/2016#technology-most-loved-dreaded-and-wanted), [2](https://insights.stackoverflow.com/survey/2017#most-loved-dreaded-and-wanted), [3](https://insights.stackoverflow.com/survey/2018/#most-loved-dreaded-and-wanted))
---
class: middle
## Bringing together two worlds
--
- systems programmers who are used to bare metal
--
- higher level language programmers who are used to abstractions
---
class: middle
## How does Rust achieve this?
--
Taking good ideas and implementing them well,
--
while staying fast, secure and reliable.
---
class: center, middle, interm
# Let's explore some functional features
---
class: center, middle, invert
# Functions
---
.p-right[Functions in Rust .small[(pt 1)]]
--
## Definition
--
```rust
fn main() {
println!("Hello World");
}
```
---
.p-right[Functions in Rust .small[(pt 1)]]
## Definition
```rust
fn head(v:Vec<u32>) -> u32 {
v[0]
}
fn main() {
let vector = vec![43, 567, 2, 34];
println!("{}", head(vector));
}
```
--
- types of variables can be inferred
--
- must annotate types of params and return values
--
- returns last expression (_no trailing `;`_)
---
.p-right[Functions in Rust .small[(pt 2)]]
## Recursion
--
```rust
fn fibonacci(nth: i32) -> i32 {
match nth {
0 => { 0 },
1 => { 1 },
n => {
fibonacci( n - 1 ) + fibonacci( n - 2)
}
}
}
fn main() {
println!("{}", fibonacci(6));
}
```
--
```bash
8
```
---
.p-right[Functions in Rust .small[(pt 3)]]
## Recursion
- Rust does [not do tail-call optimization](https://www.rust-lang.org/en-US/faq.html#does-rust-do-tail-call-optimization), so the stack is your limit!
--
- Partial application and currying à la Haskell are not possible
---
.p-right[Functions in Rust .small[(pt 4)]]
--
## Higher order functions
--
Take a function as argument and/or return a function
--
- we'd have to know the types beforehand to annotate them (required by Rust)
--
- the types of two named functions with the same signature are still different
--
How do we work around that?
---
.p-right[Functions in Rust .small[(pt 5)]]
## Closures
--
- anonymous functions
--
- can capture their environment
--
```rust
let square = |num| {
num * num
}
```
--
- type annotations are optional
---
.p-right[Functions in Rust .small[(pt 6)]]
## HOF cont.
--
_Find the sum of all odd squares that are smaller than 10,000._ .small[[haskell](http://learnyouahaskell.com/higher-order-functions#maps-and-filters), [rust](https://doc.rust-lang.org/stable/rust-by-example/fn/hof.html)]
--
```rust
fn is_odd(n: u32) -> bool {
n % 2 == 1
}
fn main() {
let upper = 10000;
let sum_of_squared_odd_numbers: u32 =
(0..).map(|n| n * n)
.take_while(|&n_squared| n_squared < upper)
.filter(|&n_squared| is_odd(n_squared))
.fold(0, |acc, n_squared| acc + n_squared);
println!("Result: {}", sum_of_squared_odd_numbers);
}
```
--
```bash
Result: 166650```
---
class: center, middle
Where do map, filter, fold etc come from?
--
### Iterators!
---
class: center, middle
### Can you talk about Iterators more?
--
Not yet!
---
class: center, middle, invert
# Structs and enums
---
.p-right[Structs and enums .small[(pt 1)]]
## Structs
--
- custom data types
--
```rust
struct Point {
x: f32,
y: f32
}
```
--
```rust
fn main() {
let p = Point { x: 3.3, y: 4.8 }
}
```
---
.p-right[Structs and enums .small[(pt 2)]]
## Enums
--
- custom data type with _enumerated_ possible values
--
```rust
enum Shape {
Circle(Point, f32),
Rectangle(Point, Point)
}
```
--
```rust
fn main() {
let circle = Shape::Circle(Point { x: 3.0, y: 4.0 }, 10.0);
}
```
---
class: center, middle, invert
# Pattern matching
---
.p-right[Pattern matching .small[(pt 1)]]
## Recap
- matching on concrete values
--
```rust
fn fibonacci(nth: i32) -> i32 {
match nth {
0 => { 0 },
1 => { 1 },
n => {
fibonacci( n - 1 ) + fibonacci( n - 2)
}
}
}
```
---
.p-right[Pattern matching .small[(pt 2)]]
## Matching on enums
--
```rust
struct Point {
x: f32,
y: f32
}
enum Shape {
Circle(Point, f32),
Rectangle(Point, Point)
}
```
---
.p-right[Pattern matching .small[(pt 3)]]
## Matching on enums
```rust
fn surface(s: Shape) -> f32 {
match s {
Shape::Circle(_, r) => { PI * r.powf(2.0) },
Shape::Rectangle(Point { x: x1, y: y1 },
Point { x: x2, y: y2 }) => {
(x2 - x1).abs() * (y2 - y1).abs()
}
}
}
```
--
- destructuring! We can access the inner values and bind them
---
.p-right[Pattern matching .small[(pt 4)]]
## Matching on enums
```rust
fn main() {
let circle = Shape::Circle(Point { x: 3.0, y: 4.0 }, 10.0);
println!("{}", surface(circle));
let rect = Shape::Rectangle(Point { x: 2.0, y: 4.0 },
Point { x: 4.0, y: 1.0});
println!("{}", surface(rect));
}
```
--
```bash
314.15927
6.0
```
---
class: center, middle, invert
# Generics and traits
---
.p-right[Generics and traits .small[(pt 1)]]
## Generics
--
An abstract stand-in for a concrete type
--
```rust
fn head<T>(v: &Vec<T>) -> &T {
v.first().unwrap()
}
```
---
.p-right[Generics and traits .small[(pt 1)]]
## Generics
An abstract stand-in for a concrete type
```rust
fn head<T>(v: &Vec<T>) -> &T {
v.first().unwrap()
}
fn main() {
let numbers = vec![43, 567, 2, 34];
let strings = vec!["hello", "foo", "world"];
println!("{}, {}", head(&numbers), head(&strings));
}
```
--
```bash
43, hello
```
---
.p-right[Generics and traits .small[(pt 2)]]
## Generics
The example would break with an empty vector.
--
```rust
let empty: Vec<u32> = vec![];
println!("{}", head(&empty));
```
--
```bash
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value'
```
---
.p-right[Generics and traits .small[(pt 3)]]
## Generics
The `Option<T>` type can help us.
--
```rust
fn head<T>(v: &Vec<T>) -> Option<&T> {
v.first()
}
```
--
```rust
fn main() {
let empty: Vec<u32> = vec![];
match head(&empty) {
Some(val) => { println!("Head is {:?}", val); },
None => { println!("No head here!"); }
}
let numbers = vec![43, 567, 2, 34];
let _num_head = head(&numbers).expect("No head!");
}
```
---
.p-right[Generics and traits .small[(pt 4)]]
## Traits
--
Traits define behaviour that types can implement
--
_Example_ `Display` trait for user facing string output
--
```rust
struct Point {
x: i32,
y: i32
}
fn main() {
let p = Point { x: 12, y: 12};
println!("{}", p);
}
```
--
```bash
println!("{}", p);
| ^ `Point` cannot be formatted with the default formatter
```
---
.p-right[Generics and traits .small[(pt 5)]]
## Traits
Implement `Display` on `Point`
--
```rust
use std::fmt;
struct Point {
x: i32,
y: i32
}
impl fmt::Display for Point {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "({}, {})", self.x, self.y)
}
}
fn main() {
let p = Point { x: 12, y: 13 };
println!("{}", p);
}
```
--
```bash
(12, 13)
```
---
.p-right[Generics and traits .small[(pt 6)]]
## Trait bounds
--
Constrain generic values with traits
--
```rust
fn exclamation<T: Display>(s: T) -> String {
format!("{}!!!!!!", s)
}
```
--
```rust
fn main() {
let p = Point { x: 12, y: 13 };
println!("{}", exclamation(p));
println!("{}", exclamation(42));
}
```
--
```bash
(12, 13)!!!!!!
42!!!!!!
```
---
class: center, middle
### Back to Iterators!
---
class: center, middle, invert
# The `Iterator` trait
---
.p-right[Iterators .small[(pt 1)]]
## Iterator trait
--
handles logic of operating on a sequence
--
## Iterators
- are thread safe
--
- are lazy
--
- adaptors convert the type of Iterator
--
- consumers kick off evaluation
---
.p-right[Iterators .small[(pt 2)]]
## Fibonacci cont.
--
```rust
struct Fibonacci {
current: i32,
index: i32
}
```
---
.p-right[Iterators .small[(pt 3)]]
## Fibonacci cont.
```rust
impl Fibonacci {
fn new() -> Fibonacci {
Fibonacci {
current: 0,
index: 1
}
}
}
```
---
.p-right[Iterators .small[(pt 4)]]
## Fibonacci cont.
```rust
impl Fibonacci {
fn new() -> Fibonacci {
Fibonacci {
current: 0,
index: 1
}
}
pub fn nth(nth: i32) -> i32 {
match nth {
0 => { 0 },
1 => { 1 },
n => {
Fibonacci::nth( n - 1 ) + Fibonacci::nth( n - 2)
}
}
}
}
```
---
.p-right[Iterators .small[(pt 5)]]
## Fibonacci cont.
```rust
impl Iterator for Fibonacci {
type Item = i32;
fn next(&mut self) -> Option<Self::Item>{
self.index += 1;
let c = Fibonacci::nth(self.index);
self.current = c;
Some(self.current)
}
}
```
---
.p-right[Iterators .small[(pt 6)]]
## Fibonacci cont.
_Among the first 10 numbers of the Fibonacci sequence, is there one odd number larger than 100?_
--
```rust
fn main() {
let f: bool = Fibonacci::new()
.take(10)
.filter(|n| { n % 2 == 1 })
.any (|n| { n > 100});
println!("{}", f);
}
```
--
```bash
false
```
---
class: center, middle, invert
# Zero cost abstractions
--
**Use abstractions _without_ additional performance cost!**
---
## We learned that Rust
--
- is an imperative language with features inspired by functional languages
--
- has a powerful type system
--
- gets its functional feel from Iterators and closures
--
- gives us higher level concepts without having to about performance
---
## Thank you!
.no-bullets[
- 🐙 https://github.com/lislis
- 📽️ https://lislis.de/talks/partial-conf-2018/]
### Resources
- [The Rust Programming Language](https://doc.rust-lang.org/book/second-edition/index.html)
- [Rust by Example](https://doc.rust-lang.org/rust-by-example/)
- [Learn you a Haskell](http://learnyouahaskell.com/chapters)
</textarea>
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