Re: [Scheme-reports] Write procedure is not backwards compatible

[Alex Shinn <alexshinn@x>]

On Sun, Jul 1, 2012 at 1:36 PM, Marc Feeley <feeley@x> wrote:
> Formal Comment
>
> Submitter's name: Marc Feeley
> Submitter's email: feeley at iro.umontreal.ca
> Relevant draft: r7rs draft 6
>
> Type: defect
> Priority: major
> Relevant section of draft: 6.13.3. Output
>
> Summary: Write procedure is not backwards compatible
>
> R7RS introduces a new output procedure called write-simple, which has
> the same semantics as the R5RS write procedure.  On the other hand,
> the R7RS write procedure handles shared structures differently than
> the R5RS.  For example :
>
>    (let ((x (list 1 2))) (write (list x x)))
>
>        displays ((1 2) (1 2)) in an R5RS system
>    and displays (#0=(1 2) #0#) in an R7RS system
>
> To preserve backwards compatibility, it is the version of the write
> procedure which uses datum labels which should have a different name.
> In fact SRFI-38 has specified the name write-with-shared-structure for
> this output procedure.  This name should be maintained since it has
> been implemented with that name in some Scheme systems.

Thanks, formal comment ticket #440 created.

We did intentionally make the safe version the
default.  The programmer should be aware of
potential infinite loops when using write-simple.

-- 
Alex

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