Mutability
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: 08 August 2024