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

[Jonathan Rees <jar@x>]

May I suggest splitting "Language changes since R5RS" into two parts:
1. What someone needs to know when porting from R5RS to R7RS (i.e. incompatible changes)
2. What someone who knows R5RS would need to learn when reading R7RS programs (mostly compatible extensions I hope)
Looks like #1 will be quite short, but the relevant items are currently spread around.

Similarly, R6RS-main incompatibilities may fall into classes, although it looks like most of the ones listed are either retractions, limitations, or "R7RS doesn't have x but it has y instead". Maybe there are extensions that aren't listed here because they're in the R5RS-diff section?

A bit more detail on the bullet "Case insensitivity is now the default" would be helpful, since it's hard to tell from reading R5RS what its position is on the subject. If there are putatively conformant case-sensitive R5RS implementations then this is an incompatible change and you should say so.

Jonathan

On Jul 1, 2012, at 5:26 PM, Jonathan Rees wrote:

> Ouch! If true, I second this comment. Backward compatibility for write is pretty important. Consider the case of running an R5RS (or R7RS) program in an R7RS implementation to generate a file that will be consumed by an R5RS implementation.
> 
> There is no mention of this incompatibility in the section "Language changes since R5RS".
> 
> Jonathan
> 
> On Jul 1, 2012, at 12:36 AM, Marc Feeley 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.
>> 
>> 
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> 
> 
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