Mutability

// This will most likely change soon.

There is 2 types of mutability:

Variable mutability(let const in JS or var val in Kotlin)
Type mutability (fields of type are immutable)

Variable mutability

x = 5 // immutable
mut y = 5 
y <- 10
y <- y inc

Type mutability

By default, all types are immutable and mutating a field creates shadow copy

type Person 
    name: String
    age: Int
    
p = Person name: "Alice" age: 32
a = p name: "Bob"
b = p age: 31

// p == Person name: "Alice" age: 32
// a == Person name: "Bob" age: 32
// b == Person name: "Alice" age: 31

To create mutable typed variable you need to mark its type as mut

p::mut Person = Person name: "Alice" age: 32
a = p name: "Bob" // ERROR
p name: "Bob" // p mutated

// p == Person name: "Bob" age: 32

Errors

So that you don't confuse the mutation of the field with the creation of a shadow copy:

// If you forgot to use result of shadow copy it will be an error

p = Person name: "Alice" age: 32
p name: "Bob" // ERROR

Conversely, if you use the result of a mutation, this is also a mistake.

p::mut Person = Person name: "Alice" age: 32
a = p name: "Bob" // ERROR

Last modified: 16 November 2024