Hash consing in Racket. (eq => equal) by definition; this makes (equal => eq).

hashcons.rkt

#lang racket/base

(provide canonicalize)

(require ffi/unsafe/atomic)

(define canonical-values (make-weak-hash))

(define (canonicalize val)
  (start-atomic)
  (define b (hash-ref canonical-values
		      val
		      (lambda ()
			(hash-set! canonical-values val (make-weak-box val))
			#f)))
  (end-atomic)
  (if b (weak-box-value b) val))

(module+ test
  (require rackunit)

  (define v1 (canonicalize (cons 1 2)))

  (let ((v2 (canonicalize (cons 1 2))))
    (check-eq? v1 v2))

  (collect-garbage)
  (check-equal? (hash-count canonical-values) 1)

  (let ((v2 (canonicalize (cons 1 2))))
    (check-eq? v1 v2))

  (set! v1 (canonicalize (cons 1 2)))

  (collect-garbage)
  (check-equal? (hash-count canonical-values) 1)

  (let ((v2 (canonicalize (cons 1 2))))
    (check-eq? v1 v2))

  (set! v1 #f)

  (collect-garbage)
  (check-equal? (hash-count canonical-values) 0))