> I've run into this multiple times as well, and would like someone (maybe me) 
> to write a package (maybe named `sugar-contract`) that provides this.

It would be neat, at least, to be able to use same double ellipsis
notation as in defproc and as we read in the documentation:

  (-> hash? any/c any/c ... ... hash?)

(Note: This would allow zero key/value pairs. But actually I think
it's desirable for (safe-hash-aet h) simply to return h.)



I don't think it's _terrible_ to raise an error manually for something
like this. You don't get the nice blame, is the drawback. For example:

    #lang racket/base

    (require racket/contract

    ; (-> hash? any/c any/c ... ... hash?)
    (define (safe-hash-set h . _kvs)
      (let loop ([h h] [kvs _kvs])
        (match kvs
          [(list) h]
          [(list* k v kvs) (loop (if (immutable? h)
                                     (hash-set h k v)
                                     (begin (hash-set! h k v) h))
          [(list v) (error
                     "expected sequence of key value ... ... but got:~n~v"

    (define imm-hash (hash 'a 1))
    (define mut-hash (make-hash '((a . 1))))

    (module+ test
      (require rackunit)

      (check-equal? (safe-hash-set imm-hash 'x 2)
                    (hash 'a 1 'x 2))
      (check-equal? (safe-hash-set imm-hash 'x 2 'y 7)
                    (hash 'a 1 'x 2 'y 7))

      (check-equal? (safe-hash-set mut-hash 'x 2)
                    (make-hash '((a . 1) (x . 2))))
      (check-equal? (safe-hash-set mut-hash 'x 2 'y 7)
                    (make-hash '((y . 7) (a . 1) (x . 2))))

      (check-exn exn:fail?
                 (λ ()
                   (safe-hash-set (hash) 'x 2 'y))
                 "safe-hash-set: expected sequence of key value ...
... but got:\n'(x 2 y)"))

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