Pampy.js: Pattern Matching for JavaScript

Pampy.js is pretty small (250 lines, no dependencies), reasonably fast, and often makes your code more readable, and easier to reason about. There is also a Python version of Pampy.

You can write many patterns

Patterns are evaluated in the order they appear.

You can write Fibonacci

The operator _ means "any other case I didn't think of". If you already use _, you can require ANY, which is exactly the same.

let {match, _} = require("pampy");

function fib(n) {
    return match(n,
        1, 1,
        2, 1,
        _, (x) => fib(x - 1) + fib(x - 2)
    );
}

You can write a Lisp calculator in 5 lines

let {match, REST, _} = require("pampy");

function lisp(exp) {
    return match(exp,
        Function,           (x) => x,
        [Function, REST],   (f, rest) => f.apply(null, rest.map(lisp)),
        Array,              (l) => l.map(lisp),
        _,                  (x) => x
    );
}
let plus = (a, b) => a + b;
let minus = (a, b) => a - b;
let reduce = (f, l) => l.reduce(f);

lisp([plus, 1, 2]);                 // => 3
lisp([plus, 1, [minus, 4, 2]]);     // => 3
lisp([reduce, plus, [1, 2, 3]]);    // => 6

You can match so many things!

let {match, _} = require("pampy");

match(x,
    3,                "this matches the number 3",

    Number,           "matches any JavaScript number",

    [String, Number], (a, b) => "a typed Array [a, b] that you can use in a function",

    [1, 2, _],        "any Array of 3 elements that begins with [1, 2]",

    {x: _},           "any Object with a key 'x' and any value associated",

    _,                "anything else"
)

You can match TAIL

let {match, _, TAIL} = require("pampy");

x = [1, 2, 3];

match(x, [1, TAIL],   (t) => t);            // => [2, 3]

match(x, [_, TAIL],   (h, t) => [h, t]);    // => [1, [2, 3])

You can nest Arrays

let {match, _, TAIL} = require("pampy");

x = [1, [2, 3], 4];

match(x, [1, [_, 3], _], (a, b) => [1, [a, 3], b]);   // => [1, [2, 3], 4]

You can nest Objects. And you can use _ as key!

pet = { type: 'dog', details: { age: 3 } };

match(pet, {details: {age: _}}, (age) => age);        // => 3

match(pet, {_: {age: _}}, (a, b) => [a, b]);          // => ['details', 3]

Admittedly using _ as key is a bit of a trick, but it works for most situations.

You can use functions as patterns

match(x,
  x => x > 3,     x => `${x} is > 3`,
  x => x < 3,     x => `${x} is < 3`,
  x => x === 3,   x => `${x} is = 3`
)

You can pass [pattern, action] array pairs to matchPairs for better Prettier formatting.

let { matchPairs, _ } = require("pampy");

function fib(n) {
  return matchPairs(
    n,
    [0, 0],
    [1, 1],
    [2, 1],
    [3, 2],
    [4, 3],
    [_, x => fib(x - 1) + fib(x - 2)]
  )
}

All the things you can match

How to install

npm install pampy