SDF dsl exercise

dsl.scm

(define (spread-combine h f g)
  (let ((n (get-arity f)) (m (get-arity g)))
    (let ((t (+ n m)))
      (define (the-combination . args)
        (assert (= (length args) t))
        (h (apply f (list-head args n))
           (apply g (list-tail args n))
        )
      )
      (restrict-arity the-combination t)
    )
  )
)

(define (restrict-arity proc nargs)
  (hash-table-set! arity-table proc nargs)
  proc
)

(define (get-arity proc)
  (or (hash-table-ref arity-table proc #f)
      (let ((a (procedure-arity proc)))
        (assert (eqv? (procedure-arity-min a)
                      (procedure-arity-max a)
                )
        )
        (procedure-arity-min a)
      )
  )
)

(define arity-table (make-key-weak-eqv-hash-table)