[plt-scheme] mutable lists/dicts?

At Sat, 10 Apr 2010 09:47:09 -0400, Carl Eastlund wrote:
> I'm not sure why mutable lists aren't dictionaries, though.

Mutable association lists don't really fit the dictionary API:
If you want to extend an empty mutable association list, then you need
`dict-set!' to return a mutable pair, rather than void. Also, the empty
mutable association list has the same representation as an association
list.

I suppose that mutable association lists could be supported if
`dict-ref!' is allowed to update existing mappings but not add new
ones, which would be like vectors. But what would be the point?