Gray Soft / Rusting

11
SEP
2014
Experimenting With Ownership

Let's use a trivial exercise to see what we can learn about ownership, moving, borrowing, and more in Rust. Here's the idea:
1. We'll allocate a list of numbers
2. We'll add one to each number in the list
3. We'll print the resulting list of numbers
This is a simple process requiring only a few lines of code:
```
fn main() {
    let mut numbers = vec![1u, 2, 3];
    for n in numbers.mut_iter() {
        *n += 1;
    }
    println!("{}", numbers);
}
```
The output is hopefully what we all expect to see:
```
$ ./one_function 
[2, 3, 4]
```
In this code there is just one variable: numbers. That variable owns a list of numbers on the heap and it's scope is limited to the main() function, which is just a way to say that the data exists for the length of that function call. Since all three steps happen in that one function call, ownership doesn't really affect us here.

To better examine what ownership really means, let's add one small twist to our exercise:
- The increment of each number in the list must happen in a separate function
Read more…
In: Rusting | Tags: Iterators, Rust & Syntax | 0 Comments
6
SEP
2014
Taking Rust to Task

Now that I've reached the point where I can get some Rust code running without asking questions in IRC every five minutes, I really wanted to play with some tasks. Tasks are the way Rust handles multiprocessing code. Under the hood they can map one-to-one with operating system threads or you can use a many-to-one mapping that I'm not ready to go into yet.

Probably one of the most exciting aspect of tasks in Rust, in my opinion, is that unsafe use of shared memory is rejected outright as a compile error. That lead me to want to figure out how you communicate correctly. (Spoiler: the same was you do in Ruby: just pass messages.)

Ready to dive in, I grossly simplified a recent challenge from work and coded it up in Rust. You can get the idea with a glance at main():
```
use std::collections::HashMap;

// ...

fn string_vec(strs: &[&'static str]) -> Vec<String> {
    let mut v = Vec::new();
    for s in strs.iter() {
        v.push(s.to_string());
    }
    v
}

fn main() {
    let mut services = HashMap::new();
    services.insert("S1".to_string(), string_vec(["A", "B"]));
    services.insert("S2".to_string(), string_vec(["A", "C"]));
    services.insert("S3".to_string(), string_vec(["C", "D", "E", "F"]));
    services.insert("S4".to_string(), string_vec(["D", "B"]));
    services.insert("S5".to_string(), string_vec(["A", "Z"]));

    let work = Work(Search::new("A".to_string(), "B".to_string()));

    let mut task_manager = TaskManager::new(services);
    task_manager.run(work);
}
```
Read more…
In: Rusting | Tags: Concurrency & Rust | 1 Comment

27

AUG
2014

Which Types to Type

I've mentioned before that I'm writing some Rust code, specifically an RPN calculator as a simple exercise. I'm going to dump the code here so we can discuss one aspect of it, but do remember that I'm very new to Rust and this code could surely be better:

use std::fmt;
use std::os;

struct Stack {
    numbers: Vec<f64>
}
impl Stack {
    fn new() -> Stack {
        Stack{numbers: vec![]}
    }

    fn is_empty(&self) -> bool {
        self.numbers.is_empty()
    }

    fn push(&mut self, number: f64) {
        self.numbers.push(number);
    }

    fn result(&self) -> f64 {
        *self.numbers.last().expect("Stack empty.")
    }

    fn add(&mut self)      { self._do_binary_operation(|l, r| l + r); }
    fn subtract(&mut self) { self._do_binary_operation(|l, r| l - r); }
    fn multiply(&mut self) { self._do_binary_operation(|l, r| l * r); }
    fn divide(&mut self)   { self._do_binary_operation(|l, r| l / r); }

    fn _do_binary_operation(&mut self, operation: |f64, f64| -> f64) {
        let r = self.numbers.pop().expect("Stack underflow.");
        let l = self.numbers.pop().expect("Stack underflow.");
        self.numbers.push(operation(l, r));
    }
}
impl fmt::Show for Stack {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let mut s = String::new();
        let mut i = self.numbers.len();
        for number in self.numbers.iter() {
            i -= 1;
            s = s.add(&format!("{}: {}\n", i, number));
        }
        s.pop_char();
        write!(f, "{}", s)
    }
}

struct Tokenizer {
    tokens: Vec<String>,
    i:      uint
}
impl Tokenizer {
    fn new(expression: &str) -> Tokenizer {
        Tokenizer{
          tokens: expression.split(|c: char| c.is_whitespace())
                            .map(|s| s.to_string())
                            .collect(),
          i:      0
        }
    }

    fn has_next_token(&self) -> bool {
        self.i < self.tokens.len()
    }

    fn next_token(&mut self) -> &str {
        if !self.has_next_token() { fail!("Tokens exhausted.") }

        let token = self.tokens[self.i].as_slice();
        self.i   += 1;
        token
    }
}

struct RPNCalculator {
    stack:  Stack,
    tokens: Tokenizer
}
impl RPNCalculator {
    fn new(stack: Stack, tokens: Tokenizer) -> RPNCalculator {
        RPNCalculator{stack: stack, tokens: tokens}
    }

    fn calculate(&mut self) -> f64 {
        while self.tokens.has_next_token() {
            let token = self.tokens.next_token();
            if !self.stack.is_empty() {
                println!("{}", self.stack);
            }
            println!("T: {}\n", token);
            match token {
                "+" => { self.stack.add(); }
                "-" => { self.stack.subtract(); }
                "*" => { self.stack.multiply(); }
                "/" => { self.stack.divide(); }
                n   => { self.stack.push(from_str(n).expect("Not a number.")); }
            }
        }
        if !self.stack.is_empty() {
            println!("{}\n", self.stack);
        }
        self.stack.result()
    }
}

fn main() {
    let     expression = os::args();
    let     stack      = Stack::new();
    let     tokenizer  = Tokenizer::new(expression[1].as_slice());
    let mut calculator = RPNCalculator::new(stack, tokenizer);
    println!("{}", calculator.calculate());
}

21

AUG
2014

Guard Clauses, Rust Style

When I'm programming in Ruby, I will often use guard clauses to prevent undesirable scenarios. For example, let's say I'm building a simple Stack:

class Stack
  def initialize
    @numbers = [ ]
  end

  def push(number)
    @numbers.push(number)
  end

  def peek
    fail "Stack underflow" if @numbers.empty?

    @numbers.last
  end
end

stack = Stack.new

ARGV.each do |number|
  stack.push(number.to_f)
end

p stack.peek

If I only want to work with numbers everywhere, I add a line like the call to fail() above. This prevents a nil from being returned from peek(), ruining my expectation that I will have numbers everywhere.

When I first started playing with Rust, I wanted to write code the same way:

use std::os;

struct Stack {
    numbers: Vec<f64>
}
impl Stack {
    fn new() -> Stack {
        Stack{numbers: vec![]}
    }

    fn push(&mut self, number: f64) {
        self.numbers.push(number);
    }

    fn peek(&self) -> f64 {
        if self.numbers.is_empty() { fail!("Stack underflow"); }

        self.numbers.last()
    }
}

fn main() {
    let mut stack = Stack::new();

    for number in os::args().tail().iter() {
        stack.push(from_str(number.as_slice()).expect("Not a number"));
    }

    println!("{}", stack.peek());
}

Rusting

11

Experimenting With Ownership

6

Taking Rust to Task

27

Which Types to Type

21

Guard Clauses, Rust Style

21

Asking Better Questions

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