feat(lang): add macro system

[eddmann]

Summary

Add a compile-time macro system to santa-lang for metaprogramming capabilities.

Description

Introduce macros that allow code transformation at compile time, enabling DSLs, code generation, and reducing boilerplate.

Current @ Usage

The @ symbol is already used for attributes (e.g., @slow for test markers). Macros could:

1. Extend the attribute syntax - @macro_name(args) for declarative macros
2. Use a different syntax - e.g., macro!, #macro, or backticks

Proposed Syntax Options

Option A: Attribute-style (extend current @ usage)

// Define a macro
@macro log_time = |expr| {
  let start = time();
  let result = expr;
  puts("Took: " + (time() - start) + "ms");
  result
};

// Use the macro
@log_time {
  expensive_computation()
}

Option B: Exclamation mark (Rust-style)

// Invoke macro
log_time! {
  expensive_computation()
}

// Define macro
macro log_time(expr) {
  quote {
    let start = time();
    let result = unquote(expr);
    puts("Took: " + (time() - start) + "ms");
    result
  }
}

Option C: Hash prefix

#log_time {
  expensive_computation()
}

Use Cases

Debug/Logging

@debug let x = complex_calculation();
// Expands to: let x = complex_calculation(); puts("x = " + x);

Conditional Compilation

@if_feature("experimental") {
  // Only included if feature enabled
}

DSLs

@html {
  div(class: "container") {
    h1 { "Hello, World!" }
    p { "Welcome to santa-lang" }
  }
}

Derive/Auto-implement

@derive(eq, hash)
let Point = |x, y| #{x, y};

Implementation Considerations

• Hygiene - Prevent accidental variable capture
• Expansion timing - Before or after parsing?
• Quoting/unquoting - How to construct and splice AST
• Error messages - Point to original source, not expanded code
• Debugging - Macro expansion visualization

References

• Rust macros: https://doc.rust-lang.org/book/ch19-06-macros.html
• Elixir macros: https://elixir-lang.org/getting-started/meta/macros.html
• Lisp macros: https://lispcookbook.github.io/cl-cookbook/macros.html