Skip to main content

Pattern Matching in ReScript

Pattern matching is an alternative to if-else. It is predominantly used in functional programming languages. In the case of ReScript, pattern matching tries to achieve even more. The official documentation states, that pattern matching in ReScript combines destructuring, a switch and a exhaustiveness check.

We'll have a look at some examples of pattern matching in ReScript.

Pattern matching destructuring

let sorted = (1, 2, 3)
let (lowest, _, highest) = sorted

lowest // 1
highest // 3

Through using the underscores, one can leave out assigning a value.

Pattern matching based on values

The most common case for pattern matching is for checking values.

let val = 1

let result = switch val {
| 1 => "Number is one"
| 0 => "Number is zero"
| _ => "anything else"


In this code example, "Number is one" will be printed. You'll get the rest.

Pattern matching based on types

Pattern matching can also be used to make decision based on types. You can imagine it like overloading functions in C++ for example.

For this example, let's set up a type which consists of three potential types - yet, only one of the three is actually assigned. In our variable "attribute",

type personAttribute =
| Name(string)
| Age(int)
| NoResult

let attribute = Age(12)

switch attribute {
| Age(attr) => Js.log("Age was provided")
| Name(attr) => Js.log("Name was provided")
| NoResult => Js.log("No proper information")

"Age was provided" will be logged.