README.md

Pampy.js: Pattern Matching for JavaScript

Pampy.js is pretty small (250 lines), 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 list [a, b] that you can use in a function",

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

    {x: _},           "any dict 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 lists and tuples

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 dicts. 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.

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 object

that value

ANY The same as _

that value

[1, 2, _] A list that starts with 1, 2 and ends with any value [1, 2, 3] 3 [1, 2, 3, 4] [1, 2, TAIL] A list that start with 1, 2 and ends with any sequence [1, 2, 3, 4] [3, 4] [1, 7, 7, 7] {type:'dog', age: _ } Any dict with type: "dog" and with an age {type:"dog", age: 3} 3 {type:"cat", age:2} {type:'dog', age: Number } Any dict with type: "dog" and with an numeric age {type:"dog", age: 3} 3 {type:"dog", age:2.3} null only null null nothing any other value undefined only undefined undefined nothing any other value

How to install

npm install pampy