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Source for ticket #440

cc

changetime

2012-10-12 05:04:56

component

WG1 - Core

description

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.

id

keywords

milestone

owner

cowan

priority

major

reporter

alexshinn

resolution

fixed

severity

status

closed

summary

Formal Comment: Write procedure is not backwards compatible

time

2012-07-02 04:12:03

type

defect

Changes

Change at time 2012-10-12 05:04:56

author

cowan

field

comment

newvalue

WG1 voted to adopt `write` (labels for cycles only), `write-simple` (no labels), and `write-shared` (full labels).

oldvalue

raw-time

1349993096915224

ticket

time

2012-10-12 05:04:56

Change at time 2012-10-12 05:04:56

author

cowan

field

resolution

newvalue

fixed

oldvalue

raw-time

1349993096915224

ticket

time

2012-10-12 05:04:56

Change at time 2012-10-12 05:04:56

author

cowan

field

status

newvalue

closed

oldvalue

accepted

raw-time

1349993096915224

ticket

time

2012-10-12 05:04:56

Change at time 2012-07-03 02:17:55

author

arcfide

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comment

newvalue

After further discussion with Marc and evaluating Scheme implementations, I think that we have a valid problem here. On the one hand, we have the desire to be able to preserve datum equivalence across a read and a write, on the other the desire to make certain writes produce valid code. Finally, we have the third desire not to make `write` unstable.

I propose that we can actually do this all with a single procedure `write` and a parameter `print-graph`. When `print-graph` is `#f` then `write` has the traditional semantics, where the behavior on cycles is undefined -- particularly, Schemes may print cycles, or they may infinite loop, but in the non-cycle case, they print the simple structure, and do not preserve shared structure. When `print-graph` is `#t` then `write` must print out the datum preserving shared structure.

The above allows `write` to be backwards compatible, but still enables implementations to implement it safely if they wish to, to handle cycles, but still gives us a way to print shared structure.

I think it is important to provide a safe, default, backwards compatible version of `write`. In that vein, I would want to require implementations that support shared structure to enable `print-graph` implicitly when a cycle is detected.

Another option is to have `write/shared` on the one hand, but then make `write` use `write/shared` whenever a cycle is detected if the implementation provides it. However, this creates some problems for folks who might want a `write` that would infinite loop. The question is whether this is worth it at all?

Using the parameter is more flexible, as it allows us to add an additional switch, where we could say that if `print-graph` is `'cycle` then it will detect cycles, but if it is `#f` it will not, and could infinite loop.

I think the question comes down to whether we want to require the handling of cycles or not.

oldvalue

raw-time

1341256675913619

ticket

time

2012-07-03 02:17:55

Change at time 2012-07-02 09:44:59

author

cowan

field

comment

newvalue

oldvalue

raw-time

1341197099196090

ticket

time

2012-07-02 09:44:59

Change at time 2012-07-02 09:44:59

author

cowan

field

owner

newvalue

cowan

oldvalue

alexshinn

raw-time

1341197099196090

ticket

time

2012-07-02 09:44:59

Change at time 2012-07-02 09:44:59

author

cowan

field

status

newvalue

accepted

oldvalue

new

raw-time

1341197099196090

ticket

time

2012-07-02 09:44:59