Stak Scheme updates

@raviqqe

October 10, 2026

Contents

• Stak Scheme
• Updates
  • Floating-point numbers
  • I/O
  • Library system
  • R7RS compatibility
• 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 implements the R7RS-small standard.

Floating-point numbers

Floating-point number literals

• Bytecode used to pack a sign, an exponent, and a mantissa into one integer.
  • The integer must fit in the 52-bit mantissa of f64.
    • Some VMs represent all numbers as f64.
  • Mantissas were limited to 38 bits (~11 decimal digits).
• Now, a mantissa is encoded as a separate integer.
  • Literals keep their full 53-bit mantissas.

Decimal representation

• number->string now prints digits up to the precision of each number.

• Other fixes
  • round with large numbers
  • Division by zero in the integer and 62-bit float configurations

I/O

I/O

• New I/O benchmarks on CodSpeed
  • In-memory ports and files
  • ASCII and UTF-8 strings
• UTF-8 encoding and decoding in ports rewritten
  • read-string from files got 3x faster.
• Bug fixes
  • peek-char and peek-u8 reordering pending input
  • UTF-8 strings in open-input-string
• (stak io) split into byte and character ((stak io utf8)) libraries

Library system

Library load paths

• (import (foo bar)) loads foo/bar.sld in library load paths.
• -I (prepend) and -A (append) options from SRFI 138
  • Both in the stak and stak-compile commands
• include in libraries resolves paths relative to their files.
• Circular imports are detected.

Library load paths

; lib/foo/bar.sld
(define-library (foo bar)
  (export hello)
  (import (scheme base) (scheme write))
  (begin
    (define (hello)
      (display "Hello, world!")
      (newline))))

; main.scm
(import (foo bar))

(hello)

stak -I lib main.scm

R7RS compatibility

R7RS compatibility

• include-library-declarations syntax (,@ for include)
• cond-expand with (library ...) checks available libraries.
  • It used to check a fixed list of library names.
• exit with integers
• Reader
  • Datum comments
  • Hexadecimal characters

(write '(1 #;2 3)) ; -> (1 3)
(write-char #\x3bb) ; -> λ

snow-chibi

• A request for snow-chibi support
  • snow-chibi install --impls=stak '(srfi 180)'
• Packages on snow-fort need the features above.
• Remaining
  • Default library directories
  • cond-expand in define-library

Compiler on Gauche

• The compiler is written in portable R7RS.
  • CI compiles all Scheme files with Chibi Scheme, Gauche, Guile, and Stak Scheme, and compares their bytecode byte by byte.
• Gauche crashed on the compiler for a year.
  • A bug in its inline-locals optimization with nested closures
  • Reported and fixed upstream on October 4
  • CI uses gosh -fno-inline-locals until the next release.

Future work

Future work

• snow-chibi support
  • Default library directories
  • cond-expand in define-library
• Big integers

Big integers?

• Investigated with AI
  • Bignums as ribs with an unused type tag
  • Arithmetic implemented in Scheme
• Options
  • An opt-in (stak bignum) library: zero cost for other programs
  • Transparent promotion with overflow checks in Scheme: 1.6-2.5x slower arithmetic
  • Primitives calling Scheme procedures on overflow: no cost on the fast path
• The numeric tower is still the final boss.

Summary

• The full R7RS-small compatibility is near!