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.
Patterns are evaluated in the order they appear.
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)
);
}
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
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"
)
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])
let {match, _, TAIL} = require("pampy");
x = [1, [2, 3], 4];
match(x, [1, [_, 3], _], (a, b) => [1, [a, 3], b]); // => [1, [2, 3], 4]
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.
match(x,
x => x > 3, x => `${x} is > 3`,
x => x < 3, x => `${x} is < 3`,
x => x === 3, x => `${x} is = 3`
)
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)]
)
}
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 |
npm install pampy