>I realize that in Smalltalk variables *point* to objects.  But in the
>above code, I assigned the constant string 'hello' to "a" and "b"
>separately -- I didn't assign a to b -- yet they still obviously point
>to the same object.  Is this what is supposed to happen, or is this
>some copy-on-write string sharing scheme gone awry?

Yes, it is supposed to happen this way.  The compiler "unifies" all equal 
literal value uses within a method.  Look at it this way:

     | a b c|
     c := 'hello'.
     a := c.
     b := c.

>If this is the
>correct behaviour, then I suppose a good Smalltalk programming rule
>would be, "don't modify an array or string object that has been
>initialized from a constant", because otherwise you may inadvertantly
>affect some other part of your program which also has a variable
>initialized from an identical constant.

Right (although literals are shared at the method-level, not 
system-wide).  If you want to modify a value from a base literal, you 
should use "copy", e.g.:

     a := 'hello' copy.
     a at: 2 put: $i.


What should be done here is to make literals immutable, and cause an 
error when modification is attempted, rather than silently modifying the 
literal.  I seem to remember someone posting an immutable-collection 
class awhile back, along with modifications to the compiler to create 
immutable strings and vectors.




     -- tim