Variables

One of the building blocks of each Y program are variables. Variables are used to store values during the runtime of the program. Declaring and instantiating a variable is straight forward:

let foo = 42;

This snippet creates a variable with the name foo and the value 42. As you can see, a variable declaration & instantiation always consists of at least these three components:

• the let keyword
• an identifier for the variable
• (technically = to perform an assignment)
• a value to assign to this variable

Mutability

By default, variables in Y are immutable. That means, once you assigned a value to a variable, you can not re-assign this variable. E.g., the following is invalid Y code:

let foo = 42;
foo = 1337; // <- invalid

If you want to re-assign a variable, you first need to declare this variable as mutable using the mut keyword:

let mut foo = 42;
foo = 1337;

This allows you to mutate variables at your own will. However, it is discouraged to just declare every variable as mutable, since this might introduce unwanted bugs to your program.