!Pampy in Star Wars

Pampy.js: Pattern Matching for JavaScript

![License MIT]()

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.

``javascript
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

`javascript
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!

`javascript
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

`javascript
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

`javascript
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!

`javascript

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


`javascript
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.

`javascript
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 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} |
| x => x > 3 | Anything greather 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``