pampy
v0.0.6
Published
Pattern Matching library for functional programming in JavaScript
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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.
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.
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