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

Pattern Example	What it means	Matched Example	Arguments Passed to function	NOT Matched Example
"hello"	only the string "hello" matches	"hello"	nothing	any other value
Number	Any javascript number	2.35	2.35	any other value
String	Any javascript string	"hello"	"hello"	any other value
Array	Any array object	[1, 2]	[1, 2]	any other value
_	Any value		that value
ANY	The same as _		that value
[1, 2, _]	An Array that starts with 1, 2 and ends with any value	[1, 2, 3]	3	[1, 2, 3, 4]
[1, 2, TAIL]	An Array that start with 1, 2 and ends with any sequence	[1, 2, 3, 4]	[3, 4]	[1, 7, 7, 7]
{type:'dog', age: _ }	Any Object with type: "dog" and with an age	{type:"dog", age: 3}	3	{type:"cat", age:2}
{type:'dog', age: Number }	Any Object with type: "dog" and with an numeric age	{type:"dog", age: 3}	3	{type:"dog", age:2.3}
x => x > 3	Anything greater than 3	5	3	2
null	only null	null	nothing	any other value
undefined	only undefined	undefined	nothing	any other value

How to install

npm install pampy