Definitions

Variables in Grain

In Grain, variables are defined using the let statement, as shown below:

let str = "Hello World"
print(str)

These variables can then be referenced later in your code.

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Patterns

Patterns allow you to destructure and bind data. Some common examples are shown below:

Tuple

let (a, b) = (1, 2)

List

let [first, ...rest] = [1, 2, 3]

A more detailed description of patterns can be found here

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Recursion

Sometimes data in Grain need to be recursive. Currently, Grain only supports recursive functions, not recursive values. Here’s the syntax:

let rec loop = () => {
	loop() // Calls itself
}

Without the rec keyword, the compiler will raise an error to help prevent unintentional recursion.

Mutual Recursion

In more complex cases, you might need mutual recursion, where two or more functions call each other. This can be done with the and keyword:

let rec greet = (times) => {
  match (times) {
    0 => print("[Grain]: Conversation Over!"),
    _ => {
      print("[Grain]: Hello!")
      reply(times - 1) // recursive call
    }
  }
} and reply = (times) => {
  print("[User]: Hi there!")
  greet(times) // recursive call
}

When run with greet(2) this produces the output:

[Grain]: Hello!  
[User]: Hi there!  
[Grain]: Hello!  
[User]: Hi there!  
[Grain]: Conversation Over!

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Mutable Variables

Sometimes you may want to make a variable mutable—allowing you to change its contents later. This can be done with the mut keyword:

let mut x = 1
print(x) // 1
x = 2
print(x) // 2

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Shadowing

A more idiomatic way to write Grain is to avoid mutable variables. This is common in functional programming but can lead to naming patterns like x, x2, x3, or data, newData. Grain solves this through name shadowing, allowing you to redefine a variable in the same scope:

let data = 1
print(data) // 1
let data = 2
print(data) // 2

Although the example above looks like it mutates data, it actually defines a new variable, potentially with a different type. This keeps names simple and helps avoid state-related bugs such as accidentally referencing old states like data instead of newData.