Couple of questions and suggestions:

Questions:
- Calling hashtable-copy with weak hashtable. How should this behave when
  GC is invoked? Should both hashtables keys/values which are not referred
  any place other than these tables be collected?
- Also hashtable-copy. How should hashtables copied with different weakness
  behave? For example, source weak hashtable A has weakness key and value,
  and copied one B has only value, then when one of the keys of A is GCed
  should B hold the key and associated value or the key should not be GCed
  in first place?

Suggestions:
- hashtable-lookup: it would be convenient to have default value which can
  be returned when the key is not found. (Then almost no difference between
  hashtable-ref, though.)
- hashtable-intern!: it would be convenient that default-proc takes the key
  as its argument so that users can derive values from the key.
- hashtable-for-each and hashtable-map!: what's the returning value of these
  procedures? Given hashtable or an unspecified value?
- I think it would be convenient to have hashtable-map which almost the same
  as hashtable-map! but returns newly created hashtable. The type of returning
  hashtable can be in sense of hashtable-copy. So simple implementation would
  be the following:
  (define (hashtable-map proc hashtable)
    (let ((r (hashtable-copy hashtable #t)))
      (hashtable-map! proc r)

      r))

Cheers,

_/_/
Takashi Kato
Email: xxxxxx@ymail.com