Swisp

A Lisp interpreter written in Swift.

Quick Start

swift run SwispCLI --repl          # Interactive REPL
swift run SwispCLI file.lisp       # Run a file
echo '(+ 1 2)' | swift run SwispCLI -  # Pipe stdin

Features

• BigInt arithmetic — integers auto-promote to arbitrary precision on overflow
• Reader macros — register custom syntax via (set-reader-macro-char char fn)
• Quasiquote — `(a ,x ,@list) expanded by a Lisp reader macro
• Rest parameters — (lambda (a b . rest) ...)
• apply / funcall — spread lists into function calls
• setq — multiple-pair assignment
• require / provide — load Lisp libraries
• CPS evaluator — continuation-passing style with trampoline
• Tail-call optimization — self-recursive tail calls reuse the environment frame in O(1) memory
• Clean errors — no Swift stack traces for Lisp errors
• REPL — with multi-line expression support

Examples

;; BigInt — no overflow
(fact 100)

;; Rest params
(define (sum . nums) (apply + nums))

;; Reader macro for lambda
(set-reader-macro-char "!" (lambda (c) (list 'lambda (read) (read))))
(map !(x) (* x x) '(1 2 3))  ; → (1 4 9)

;; Quasiquote
`(a ,@(map (lambda (n) (* n n)) '(1 2 3)))

;; Tail-recursive — O(1) memory, 5M+ calls
(define (count n max)
  (if (= n max) n (count (+ n 1) max)))
(count 0 5000000)  ; → 5000000

Builtins

Arithmetic: + - * /
Comparison: = < > <= >=
Lists: cons car cdr list null? eq? equal?
Predicates: number? symbol? string? boolean? pair?
Functions: apply funcall
I/O: print display newline read read-char peek-char
Reader: set-reader-macro-char
System: error require provide

Special forms: quote if lambda define define-macro set! setq begin quasiquote require

License

MIT