visible hash tables

[Dave.Touretzky@C.CS.CMU.EDU]

It bothers me that hash tables cannot be made "visible" data structures, i.e.,
there is no pretty printer switch you can set to see what's inside them.
Compare this with:

 Vectors:  contents displayed in #() notation if *PRINT-ARRAY* is T.

 Arrays:  contents displayed in #nA() notation if *PRINT-ARRAY* is T.

 Structures:  normally displayed in #S notation.  As previously discussed,
  there should be a simple way to select #<> notation for structures, e.g., 
  if a flag named *PRINT-STRUCTURE* is NIL.

In teaching beginning to intermediate level Lispers to use hash tables, I find
my job would be a lot easier if they could see inside the things, just as they
can see inside structures and vectors.  So what about a convention for printing
hash tables?  Here is a suggestion:

  (setq a (make-hash-table))    ;make a hash table
  (setf (gethash 'foo a) 37)    ;put some stuff in it
  (setf (gethash 'bar a) 42)    ;put some more stuff in it

  (setf *print-hash* t)		;turn on the magic printer flag

  a ==>  #65H(EQL (FOO . 37) (BAR . 42))

The basic notation is:

  #nH(type (key1 . value1) (key2 . value2) ...)

where "n" is the size of the hash table and "type" is the type, such as
EQL.  The ordering of the dotted pairs is arbitrary.  Comments?

-- Dave
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