Unicode in Stak Scheme

@raviqqe

November 15, 2025

Contents

• Stak Scheme
• Progress
  • Backtrace on errors
  • Unicode in (scheme char)
• 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.

Progress

• Backtrace on errors
• Unicode in (scheme char)

Backtrace on errors

• On the VM, instructions are in a linked list.
• On a call instruction, the VM calls its procedure storing a return address of the current call instruction.
  • We cannot go back when it points to the next instruction.

Example

Source code:

(import (scheme base))

(define (foo)
  (error "Oh, no!" 42)
  #f)

(let ()
  (foo)
  #f))

Output:

Oh, no! 42 [error foo eval #f]

Unicode in (scheme char)

• Stak Scheme already supports Unicode in I/O.
  • i.e. UTF-8 encoding
• Now, its (scheme char) library also supports Unicode.
• Unicode defines multiple tables for character properties.
  • e.g. categories, and case mappings
• The tables can be fairly large...
  • One of Stak Scheme's goals is small memory footprints.

Encoding Unicode tables

Example: Upper to lower case mapping

1. Parse a table.

   ; A -> a, B -> b, C -> c, ... Z -> z
   ((65 97) (66 98) (67 99) #| ... |# (90 122))
   

2. Calculate differences between rows.
   • Small integers are encoded into small bytes in bytecode encoding.

   ((65 97) (1 1) (1 1) #| ... |# (1 1))
   

3. Apply run-length encoding.

   ((65 97) (24 . 1))
   

Asymmetric mapping

(import (scheme base) (scheme char) (scheme write))

(write
  (map
    (lambda (char) (cons char (char->integer char)))
    (list #\ß (char-upcase #\ß) (char-downcase #\ẞ))))

> stak ~/foo.scm # same for guile and gosh
((#\ß . 223) (#\ß . 223) (#\ß . 223))
> chibi-scheme ~/foo.scm
((#\ß . 223) (#\ẞ . 7838) (#\ß . 223))

Asymmetric mapping

• UnicodeData file format

  Uppercase mapping
  These mappings are always one-to-one, not one-to-many or many-to-one.

• ß - Wikipedia

  Because ⟨ß⟩ had been treated as a ligature, ... it had no capital form in early modern typesetting. ... A capital was first seriously proposed in 1879, but did not enter official or widespread use.

Future work

• LZSS bytecode compression
• Soft float in Scheme
• DSW garbage collection