Scrap.js

Scrap.js let's you declaratively define data types that come with rich manipulation and traversal functionality built right in without the boilerplate.

Project Status

Experimental, expect the API to change (but will follow semver when it does).

Use

Install:

npm install @scrap-js/scrap

Use:

import scrap from '@scrap-js/scrap';
import { reduceSum } from '@scrap-js/scrap/transformers';

let { Node, Leaf } = scrap`
data Node { left: Node | Leaf, right: Node | Leaf }
data Leaf { data: any }
`;

let tree = Node(
  Node(Leaf(1), Leaf(10)),
  Leaf(6)
);

let sum = reduceSum(tree,
                    Leaf.case(({ data }) => data));
// sum === 17;

Note at the moment this project uses ES modules exclusively so you'll need a recent version of node or a bundler.

Documentation

Scrap.js has two main components:

• a declarative DSL for defining data types inside template literals
• a recursion scheme API for performing declarative transformations over the data

Scrap DSL

Data types are defined within scrap template literals like so:

import scrap from '@scrap-js/scrap';

let { Pair } = scrap`
data Pair { left: number, right: number }
`;

The syntax data Pair { left: number, right: number } defines a data type called Pair with two fields left and right both of type number. The result of invoking scrap is an object with data constructors for all the data declarations within the template literal.

As the name implies, data constructors allow you to construct object from your data types. Note that data constructors are not JavaScript class constructors and should be invoked without the new keyword:

let p = Pair(1, 2);
p.left === 1;
p.right === 2;

Note that the order of the arguments to the constructor will match the lexical order of fields in the data declaration.

You can check if some value was made by a data constructor via the static is predicate:

let p = Pair(1, 2);

Pair.is(p) === true;
Pair.is([1, 2]) === false;

Data Types

A data type field can have the types:

• any type: any
• the JavaScript base types: number, string, boolean, ...
• an Array type: [<type>]
• the union type: <type 1> | <type 2>
• a custom data type defined in another data declaration

Mixins

A data declaration can "mixin" fields from another declaration:

data Base { a: number }
data Derived { b: string, ...Base }

This is the equivalent of writing:

data Base { a: number }
data Derived { b: string, a: number }

Scrap API

Scrap.js comes with two main kinds of manipulation functions (with some variants):

• reconstruct - take a data structure and rebuild it with (potentially) modifications
• reduce - take a data structure and "summarize" it into a different value

These manipulation functions combo with a static function on each data constructor called case (described below).

Using a tree structure for our running example:

import scrap from '@scrap-js/scrap';

let { Node, Leaf } = scrap`
data Node { left: Node | Leaf, right: Node | Leaf }
data Leaf { data: number }
`;

let tree = Node(
  Node(Leaf(1), Leaf(10)),
  Leaf(6)
);

reconstruct(data, ...cases)

Reconstruct data bottom-up, matching and transforming each data type by running cases over them.

For example, let's say we want to increment the number in each leaf by one:

import { reconstruct } from '@scrap-js/scrap/transformers.mjs';

let resultTree = reconstruct(tree,
  Leaf.case(({ data }) => Leaf(data + 1))
);

reconstruct will walk tree bottom-up and apply the function passed to Leaf.case to each Leaf object it encounters replacing the object with the result of the function application. Any non-Leaf objects are left alone (or reconstructed if their children were modified).

Alternatively, say we want to replace all right nodes with -1:

let resultTree = reconstruct(tree,
  Node.case(({ left, right }) => Node(left, Leaf(-1)))
);

Or combining it all together:

let resultTree = reconstruct(tree,
  Leaf.case(({ data }) => Leaf(data + 1)),
  Node.case(({ left, right }) => Node(left, Leaf(-1)))
);

Variants:

• reconstructTopDown - reconstruct top-down instead of bottom-up
• reconstructBottomUp - reconstruct bottom-up instead of top-down
• reconstruct - an alias of reconstructBottomUp

reduce(data, empty, concat, ...cases)

Reduce data bottom-up. Run cases over each data type. concat is used to combine the results of cases and empty is used when no cases match a data type.

An example of summing all the numbers in a tree should be more clear:

let sum = reduce(tree, 0, (l, r) => l + r,
                Leaf.case(({ data }) => data));

The case Leaf.case(({ data }) => data) extracts the number from each Leaf. Note the type of the case function here is Leaf -> number whereas when case is used in reconstruct the type is Leaf -> Leaf.

The concat function is used to combine (sum) results from each case and the empty value 0 is used as the default (whispers: monoid).

Variants:

• reduceSum - like reduce but with pre-set empty as 0 and concat as +
• reduceConcat - like reduce but pre-set empty as [] and concat as Array.prototype.concat

Why the name?

From the excellent paper "Scrap your boilerplate".