(I'm using my suggested terminology here.)

Being a rectangular array should be orthogonal to being a mutable array,
which should be orthogonal to being a safe array.  These effects can be
accomplished by allowing the setter and checker of a storage class to
be #f rather than a procedure, making the array immutable and unsafe
respectively.  Furthermore, allow the generalized array creator to
take an optional setter as well as a domain and a getter.  In that way,
we wind up with arrays and a single subtype, rectangular arrays.

In addition, a standardized sparse-storage-class would be a Good Thing.
This would use a hash table or similar object as its backing store.

--
John Cowan          http://www.ccil.org/~cowan        xxxxxx@ccil.org
There are three kinds of people in the world:
those who can count, and those who can't.