Pattern matching is a powerful concept used with Functional Programing. It is actually a mechanism for checking a value against a pattern. A successful match can also deconstruct a value into its constituent parts.
Lets see some examples
Elixir
1
2
3
4
5
6
7
8
iex> [a, b, c] = [:hello, "world", 12]
[:hello, "world", 42]
iex> a
:hello
iex> b
"world"
iex> c
42
It should not be mistaken for multiple assignment in Ruby. In pattern matching the = operator checks whether the values get matched as well.
which means
1
2
3
4
5
6
7
8
9
10
11
12
13
14
iex> [a, b] = [] #left and right number of values are different
** (MatchError) no match of right hand side value: []
iex> {:ok, result} = {:ok, 13}
{:ok, 13}
iex> result
13
iex> {:ok, result} = {:error, :oops} #left side will only match the right side when the right side is a tuple that starts with the atom
** (MatchError) no match of right hand side value: {:error, :oops}
iex> {a, b, c} = [:hello, "world", 42] #mistatched data type
** (MatchError) no match of right hand side value: [:hello, "world", 42]
the above all will throw an error.
In lists we can also do
1
2
3
4
5
6
iex> [head | tail] = [1, 2, 3]
[1, 2, 3]
iex> head
1
iex> tail
[2, 3]
Which is quite useful for easy destructure and prepending items to a list as well.
1
2
3
4
iex> list = [1, 2, 3]
[1, 2, 3]
iex> [0 | list]
[0, 1, 2, 3]
Javascript
In JS, pattern matching concept is indroduced via Array and Object Destructuring.
For example:
1
2
3
const colors = ['red', 'orange', 'green'];
const [red, orange, green] = vehicles;
Also
1
2
3
4
5
6
7
8
9
10
11
12
13
const vehicleOne = {
brand: 'Ford',
model: 'Mustang',
type: 'car',
year: 2021,
color: 'red'
}
myVehicle(vehicleOne);
function myVehicle({type, color, brand, model}) {// Destructured here
const message = 'My ' + type + ' is a ' + color + ' ' + brand + ' ' + model + '.';
}
which is quite useful.