Tables

[Jonathan Young <young-jonathan@YALE.ARPA>]

I would like to see some discussion about desirable features for
the T table package.

Recall that the current ("Released", although it isn't in the manual)
table package supports the following operations:

        (MAKE-TABLE tag)        => table
        (TABLE-ENTRY table key) => value or nil if none
                                        (settable)
        (SET-TABLE-ENTRY table key value) => undefined
                                (adds an association)

Now, the wish list.

Clearly, it is desirable to have hash tables where the hash function
and comparision function are other than POINTER-HASH (or some other
hack) and EQ?.  Obvious examples are <STRING-HASH, STRING-EQUAL?>,
<SYMBOL-HASH, EQ?>, and <TREE-HASH, ALIKEV?>.

Weak Tables.  A table is a list of associations - pairs of
keys and values.  If the key is garbage, i.e., is no longer accessible,
I would like the association (i.e. the pointer to the value) to go away.  
This is not implementable in the current scheme.

Table Walking.  It is desirable (occasionally) to apply a procedure to
each <key,value> pair in a table.  This is possible in the current 
scheme only if you put a layer on top of the table package.

Generators.  I often write the following idiom:

        (or (table-entry table key)
            (set (table-entry table key) (make-value key)))

This would be eliminated by the provision of a generator procedure
to be called in the case when there is no value associated
with the key in a call to TABLE-ENTRY.

Before someone goes out and blindly writes a new table package,
I'd like to hear more feedback.  Are these idioms you use a lot?
Are there other ideas you really need?  Is this even the proper 
abstraction? 

                                --- Jonathan



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