[mapcars]

No need to go anywhere :)

  >> words: keys-of system/words
  >> types: collect [forall words [if datatype! = type? get/any words/1 [keep words/1]]]
  >> probe types
  [datatype! unset! none! logic! block! paren! string! file! url! char! integer! float! word! set-word! lit-word! get-word! refinement! issue! native! action! op! function! path! lit-path! set-path! get-path! routine! bitset! point! object! typeset! error! vector! hash! pair! percent! tuple! map! binary! time! tag! email! handle! image! event!]

[greggirwin]

Even easier:

  help datatype!

[pekr]

It can't be that easy, or can it? :-)