scrapmetal: Scrap Your Rust Boilerplate

Generic transformations, queries, and mutations for Rust without the boilerplate.

A port of some of the ideas and code from "Scrap Your Boilerplate: A Practical Design Pattern for Generic Programming" by Lämmel and Peyton Jones to Rust.

⚠ Depends on the specialization nightly Rust feature. ⚠

Say we work on some software that models companies, their departments, sub-departments, employees, and salaries. We might have some type definitions similar to this:

pub struct Company(pub Vec<Department>);

pub struct Department(pub Name, pub Manager, pub Vec<SubUnit>);

pub enum SubUnit {
    Person(Employee),
    Department(Box<Department>),
}

pub struct Employee(pub Person, pub Salary);

pub struct Person(pub Name, pub Address);

pub struct Salary(pub f64);

pub type Manager = Employee;
pub type Name = &'static str;
pub type Address = &'static str;

One of our companies has had a morale problem lately, and we want to transform it into a new company where everyone is excited to come in every Monday through Friday morning. But we can't really change the nature of the work, so we figure we can just give the whole company a 10% raise and call it close enough. This requires writing a bunch of functions with type signatures like fn(self, k: f64) -> Self for every type that makes up a Company, and since we recognize the pattern, we should be good Rustaceans and formalize it with a trait:

pub trait Increase: Sized {
    fn increase(self, k: f64) -> Self;
}

A company with increased employee salaries is made by increasing the salaries of each of its departments' employees:

impl Increase for Company {
    fn increase(self, k: f64) -> Company {
        Company(
            self.0
                .into_iter()
                .map(|d| d.increase(k))
                .collect()
        )
    }
}

A department with increased employee salaries is made by increasing its manager's salary and the salary of every employee in its sub-units:

impl Increase for Department {
    fn increase(self, k: f64) -> Department {
        Department(
            self.0,
            self.1.increase(k),
            self.2
                .into_iter()
                .map(|s| s.increase(k))
                .collect(),
        )
    }
}

A sub-unit is either a single employee or a sub-department, so either increase the employee's salary, or increase the salaries of all the people in the sub-department respectively:

impl Increase for SubUnit {
    fn increase(self, k: f64) -> SubUnit {
        match self {
            SubUnit::Person(e) => {
                SubUnit::Person(e.increase(k))
            }
            SubUnit::Department(d) => {
                SubUnit::Department(Box::new(d.increase(k)))
            }
        }
    }
}

An employee with an increased salary, is that same employee with the salary increased:

impl Increase for Employee {
    fn increase(self, k: f64) -> Employee {
        Employee(self.0, self.1.increase(k))
    }
}

And finally, a lone salary can be increased:

impl Increase for Salary {
    fn increase(self, k: f64) -> Salary {
        Salary(self.0 * (1.0 + k))
    }
}

Pretty straightforward.

But at the same time, that's a whole lot of boilerplate. The only interesting part that has anything to do with actually increasing salaries is the impl Increase for Salary. The rest of the code is just traversal of the data structures. If we were to write a function to rename all the employees in a company, most of this code would remain the same. Surely there's a way to factor all this boilerplate out so we don't have to manually write it all the time?

Enter scrapmetal:

// Imports
#[macro_use]
extern crate scrapmetal_derive;
extern crate scrapmetal;
use scrapmetal::{Everywhere, Transformation};

// Add derive(Term) to type definitions
#[derive(Term)]
pub struct Company(pub Vec<Department>);
// Etc...

// Define the `increase` transformation
let increase = |s: Salary| Salary(s.0 * 1.1);
let mut increase = Everywhere::new(Transformation::new(increase));

// Use the `increase` transformation
let new_company = increase.transform(old_company);

Nothing more required!