Version: 1.1.0

Initialization state

Mojo tracks whether a name is initialized the way it tracks types. A name is either ready to use or it is not.

Essentials

• A var with an immediate value starts initialized.
• A var with only a type starts uninitialized.
• Every branch must initialize before the name is read.
• Transferring a value with ^ leaves the source uninitialized.
• Early returns still have to cover the paths that read the name.

Example

def make(flag: Bool) -> Int:
    var n: Int
    if flag:
        n = 1
    else:
        n = 2
    return n

Keep in mind

This state is a compile-time fact. There is no runtime null hiding inside an uninitialized Int.