evaling macros in Stak Scheme

@raviqqe

June 2, 2024

Contents

• Stak Scheme
• Macros in Scheme
• Implementation in Stak Scheme
• Demo
• Future work

Stak Scheme

• A bytecode compiler and virtual machine (VM) for Scheme
  • The compiler is written in Scheme.
  • The VM is written in Rust.
• It aims to support R7RS-small.

Macros in Scheme

• Macros are defined in source codes.
  • Usually in (define-library) forms.
• Macros in Scheme can be expanded at compile time.
  • Stak's compiler does that.

(define-syntax or
  (syntax-rules ()
    ((_)
      #f)

    ((_ test)
      test)

    ((_ test1 test2 ...)
      (let ((x test1))
        (if x x (or test2 ...))))))

Macros in eval

• The eval procedure evaluates S-expressions.
  • The expressions can be primitive values, procedure calls, or macro expansions.
• No macros or their information at runtime when they are expanded at compile time!
• We need pass macros into runtime codes in some way.

Implementation of macros in eval in Stak Scheme

($$macros) primitive

• Used in Scheme source codes.
• Expanded by a compiler into macro rules.
  • Macro rules are represented by lists, symbols, and literals.
• At runtime, macros are compiled to macro transformers of procedures.

Implementation of macros in eval in Stak Scheme

Macro expansion in eval

• The eval procedure expands all macros in a given expression first.
• Then, it compiles the expression into a temporary procedure and calls it.

Demo

Future work

• Deduplication of codes between a compiler and the (scheme eval) library

Summary

• Building macros in eval is fun!