[plt-scheme] Mutable and immutable pairs in PLT Scheme

From: Robby Findler (robby at cs.uchicago.edu)
Date: Tue Nov 25 18:26:31 EST 2008

Please read Matthew's blog post on the subject. It contains the answer you seek.



On 11/25/08, Thomas Chust <chust at web.de> wrote:
> Hello,
> the discussion about R6RS support in PLT Scheme brought the topic of
> immutable pairs in PLT's default Scheme dialect to my attention again.
> Basically, I think having immutable pairs as the default is a good
> thing, but I start wondering why mutable and immutable pairs are
> completely disjunct types. Other datatypes, like strings, vectors and
> hash tables, that also exist in mutable and immutable variants in PLT
> Scheme, are not disjunct, but the pairs, maybe the most prominent
> datatype(s) of Scheme, behave differently. Is there any clever rationale
> behind this?
> I would actually have expected that the following conditions all hold
>   (pair? (mcons 1 2))
>   (pair? (cons 1 2))
>   (not (immutable? (mcons 1 2)))
>   (immutable? (cons 1 2))
> However, mutable pairs are not pairs, as far as pair?, car and cdr are
> concerned and immutable pairs are not immutable, as far as immutable? is
> concerned. This seems totally counterintuitive to me!
> So unless there is a deeper reason for the disjunctness of the mutable
> and immutable pair types, I suggest that the type of mutable pairs
> should be made a subtype of the type of pairs, and consequently car and
> cdr could be used to access mutable pairs just like immutable ones but
> set-car! and set-cdr! could only be used on mutable ones. mpair? or
> immutable? could be used to distinguish the types.
> As far as I can see, my proposal would not introduce problems in most
> existing code, the exception being code that explicitly relies on
> immutable pairs being declared mutable by immutable? or code that uses
> exception handling to catch expected parameter type errors -- both
> things that I would consider bad programming practice. The proposal
> would however greatly improve compatibility between libraries written in
> R6RS and those written in PLT Scheme, since one could always work with
> pairs returned from R6RS on the PLT Scheme side without problems and, in
> many cases one could also pass PLT Scheme's immutable pairs to R6RS code
> without causing trouble. And last but not least, I think this behaviour
> would just feel more natural.
> What do you think about it?
> cu,
> Thomas
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