Mutability design (`mut`)

[Sven Nilsen]

Dyon uses mut to declare mutability. This appears in front of a function argument declaration or when calling a function that mutates the argument.

• foo(mut a) mutates the argument a
• foo(mut a) has function name foo(mut)
• foo(mut a, b) has function name foo(mut,_)
• foo(a, b) has function name foo

Local variables are mutable.

Mutability is not part of a type, but added to the function name inside parentheses, e.g.(mut,_). When adding an external function, the mutability information must be added as part of the function name.

Mutability propagates for arguments:

fn foo(mut a) { ... }

// `bar` requires `mut a` because it calls `foo(mut a)`.
fn bar(mut a) { foo(mut a) }

Multiple functions can use the same name if they have different mutability information. This reduces the need for "get", "set" and "mut" in function name:

fn title(window: 'return) -> { ... }

fn title(mut window, val) { ... }

t := title(window)
title(mut window, "hello world!")

This is designed for:

• Explicitly declare when a function mutates a variable
• Improve readability and maintenance
• Allow a function name to be reused for different mutability patterns