Refactoring Rust: Primitive Obsession

Fast smelly code is still smelly

Posted by Ian Whitney on August 31, 2016

Thanks to their small, focused nature problems in Exercism tend to focus on language primitives. Transform this string into a different string, manipulate this number, etc. In the bob exercise students return a string based upon a string input. And in leap students return a boolean based on an integer input.

Students tend to rely on primitives when solving these problems. The example solutions for bob and for leap are indicative. They accept primitive input, use primitives to solve the problem and return primitive results.

This is, in short, primitive obsession. And for people just starting to learn Rust (or any language) primitive obsession is fine. Beginners on the Dreyfus model aren’t yet modeling domains or complex concepts; they are still learning the techniques these problems exercise: syntax, primitives, control flow, etc.

But as students progress through the exercises their skills will evolve and they’ll be able ponder maintainability and design. The question will arise – how do I write good Rust code?

I certainly don’t have the answers to that question, and in the Rust track we’re barely starting to think about how to help students answer that question. Should exercises actively discourage certain code smells, like Primitive Obsession? What would that discouragement even look like? But the discussion has led me to start thinking about what code smells look like in Rust and how we can apply standard Refactoring patterns to remove them.

As always, this is easier to understand with an example. Let’s look at Primitive Obsession in one of the most recent additions to the Rust exercise track: Bracket Push.

The problem is straight-forward: return true if the brackets in a string are balanced (all open brackets are closed in the correct order), and return false if they are not. There are a few different ways of solving it; I started with the following:

pub fn are_balanced(input: String) -> bool {
    let mut unmatched_brackets: Vec<char> = Vec::new();

    for current_bracket in input.chars() {
        if let Some(most_recent_unmatched) = unmatched_brackets.pop() {
            let is_matched = match (most_recent_unmatched, current_bracket) {
                ('[', ']') => true,
                ('{', '}') => true,
                ('(', ')') => true,
                _ => false,
            };
            if !is_matched {
                unmatched_brackets.push(most_recent_unmatched);
                unmatched_brackets.push(current_bracket);
            }
        } else {
            unmatched_brackets.push(current_bracket);
        }
    }

    unmatched_brackets.is_empty()
}

fn main() {
    assert!(are_balanced("{[]}".to_string()));
    assert!(!are_balanced("[{]}".to_string()));
}
Run

There are better ways to structure the logic, sure. But set that aside for a minute and let’s focus on the primitives. In the above code I count five of them:

  • String*
  • char
  • boolean
  • Vector*
  • Tuple

* The Rust book doesn’t list these in the Primitive types section, but I’m fine with thinking of them as primitives. It’s a fuzzy line.

The are_balanced function must accept a string and return a boolean to satisfy the tests. But inside the are_balanced function we can do whatever we want, and yet what I chose to do was use a bunch of primitives.

The primitives inside the function have satisfied all of the criteria of the Primitive Obsession smell: they were easy for me to use and they passed the tests. However, they obscured my code’s intent and made it hard to change. I tried to provide some useful variable names but I still find that I have to read the entire function, slowly, before I understand what it’s doing. This is not code that is easy to understand; and poorly understood code is rarely well maintained code.

So, let’s fix it! Checking with Martin Fowler the official fix for Primitive Obsession is to replace the primitive with an object. Well, Rust doesn’t have objects so we can’t do that. But Rust still gives us all the tools we need to refactor away these primitives – Structs and Traits will do most of the heavy lifting. If we replace our primitives with structs and traits that model the domain we will improve our code’s clarity and maintainability. Where to start? Since the first primitive our function deals with is the String, let’s start there.

We’re not going to change the test, because refactoring should not change the behavior of existing code. Our function will still accept a string, but we can convert it to domain-specific Struct inside the function.

What should we call this Struct? Naming is hard, as always. Since it contains a bunch of brackets let’s just call it Brackets for now. Maybe a better name will occur to us later. Brackets will start off containing our input string.

struct Brackets {
  input: String,
};

How do we create our Brackets inside are_balanced?. We could do this:

pub fn are_balanced(input: String) -> bool {
  let brackets = Brackets { input: input };
  //...

And that’s…OK. But it’s not idiomatic. We’re trying to convert a String to a Brackets and Rust has a standard way to convert types – the From trait. You may have seen this trait before in the commonly used String::from(....) function. But you can implement From for any type!

struct Brackets {
  input: String,
};

impl From<String> for Brackets {
  fn from(input: String) -> Self {
    Brackets { input: input }
  }
}

Wait! That’s pretty much exactly the same thing we did in the last example. What’s the benefit of implementing From? First, we’re both following a Rust idiom and establishing a example for our code. And this example will guide us if (read: when) we want to create a Brackets from something else? A vector? A &str? Time will tell, but we’ve already established the protocol to follow whatever happens.

All of this is good…but it doesn’t help us. Our code still relies on the behavior of a string primitive:

pub fn are_balanced(input: String) -> bool {
    //..
    for current_bracket in input.chars() {
    //..
    }
}

Our Brackets doesn’t implement chars(). We can make it do so:

use std::str::Chars;
impl Brackets {
  fn brackets(&self) -> Chars {
    self.input.chars()
  }
}

That feels unsound to me, though I can’t put a finger on why. I suspect this shortcut will come back and bite us later. Let’s be a little more thorough and idiomatic.

chars returns an iterator and – just like the From trait and type conversion – Rust has a standard way of adding iterators to your Structs, IntoIterator

impl IntoIterator for Brackets {
  type Item = char;
  type IntoIter = ::std::vec::IntoIter<char>;

  fn into_iter(self) -> Self::IntoIter {
    self.input.chars().collect::<Vec<char>>().into_iter()
  }
}
// Note: I mostly copied this code from The Book; there may be a better way of implementing this Trait for Brackets.
// Update: Ms2ger showed me how to implement this without a vector:
//   https://github.com/IanWhitney/designisrefactoring/pull/22#discussion_r77125688

With this done, we can use Brackets throughout are_balanced

struct Brackets {
    input: String,
}

impl From<String> for Brackets {
    fn from(input: String) -> Self {
        Brackets { input: input }
    }
}

impl IntoIterator for Brackets {
    type Item = char;
    type IntoIter = ::std::vec::IntoIter<char>;

    fn into_iter(self) -> Self::IntoIter {
        self.input.chars().collect::<Vec<char>>().into_iter()
    }
}

pub fn are_balanced(input: String) -> bool {
    let brackets = Brackets::from(input);
    let mut unmatched_brackets: Vec<char> = Vec::new();

    for current_bracket in brackets.into_iter() {
        if let Some(most_recent_unmatched) = unmatched_brackets.pop() {
            let is_matched = match (most_recent_unmatched, current_bracket) {
                ('[', ']') => true,
                ('{', '}') => true,
                ('(', ')') => true,
                _ => false,
            };
            if !is_matched {
                unmatched_brackets.push(most_recent_unmatched);
                unmatched_brackets.push(current_bracket);
            }
        } else {
            unmatched_brackets.push(current_bracket);
        }
    }

    unmatched_brackets.is_empty()
}

fn main() {
    assert!(are_balanced("{[]}".to_string()));
    assert!(!are_balanced("[{]}".to_string()));
}
Run

By implementing a Struct and two traits we’ve:

  • Given a name to our previously-nameless String
  • Introduced an idiomatic way of creating our Brackets
  • Used idiomatic Rust to show that Brackets can be iterated
  • Made our code longer

No doubt about it, our code is longer. But it also more clearly illustrates what it’s doing, and I’m OK with that tradeoff.

That’s enough for now. Of our original list of five primitives there remain 3 that I would like to remove. Next time we’ll tackle the Tuple.

If you want to comment, question, complain, &c. then you can do so on twitter, at ian@ianwhitney.com, or by leaving comments on the pull request for this post.