Awesome
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.
<kbd> <img src="https://github.com/santinic/pampy.js/raw/master/imgs/slide1.png" width="700"> </kbd>You can write many patterns
Patterns are evaluated in the order they appear.
<kbd> <img src="https://github.com/santinic/pampy.js/raw/master/imgs/slide2.png" width="700"> </kbd>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