mutable/immutable

[Michael R. Blair <ziggy@martigny.ai.mit.edu>]

Is this an oversight or is there some subtle rationale behind the following:

 literals are constants and therefore immutable.
 other constructed composite data are mutable.

 There is/are no predicate(s) to test mutability.

E.g, (list 1 2 3) is mutable whereas '(1 2 3) is not.


     We have (pair? x) and (list? x) but not (mutable? x)
     [or (mutable-pair? x) or (mutable-list? x) etc]

I am working on code that traverses aggregate data attempting to live in-line
marking to avoid the cyclic data abyss. I am thwarted by stumbling across
immutable data. I'd rather not overflow my heap by being forced to maintain an
external marker trail.

This is a nuisance in practice.

-------------------------------------------------------------------------------
ziggy@lcs.mit.edu    Michael R. Blair   MIT Laboratory for Computer Science
(617) 253-8576 [O]         -.           545 Technology Square --- Room 434
(617) 577-1167 [H]          /\.         Cambridge, MA   02139