I am `gen:custom-write` and `make-constructor-style-printer` on an object
graph that may contain cycles. Within my custom write procedure, I use
`parameterize` to register an object into dictionary and later in the
writing process I can check that dictionary, detect the cycle, and do more
custom write logic.
But I have noticed that sometimes that my `parameterize` has gone out of
scope by the time the printer gets deeper into the object graph. I was able
to work around this issue by using another port, then copying the result to
the real port like this:
(define-syntax-rule (write-now proc x port mode)
(let* ([alt-port (open-output-string)]
[_ (proc x alt-port mode)]
[str (get-output-string alt-port)])
(write-string str port)))
This is better than just doing `(proc x port mode)` because it always
prints the string that I expect. However, it breaks pretty printing. I've
noticed that the built-in pretty printing logic will sometimes hit the same
leaf object multiple times, as if it is testing to see how wide the result
is and backing up and trying again. Perhaps it can also delay evaluation?
That would explain why my `parameterize` has ended.
Is there a way to use `parameterize` within custom write logic and keep
pretty printing working? Is there a better approach I should take?
Additional Information
===
https://github.com/default-kramer/morsel/blob/master/morsel-lib/private/essence/base-query-printer.rkt#L22
The `write-now` macro, as written, is "correct but unpretty". I can change
it to "incorrect but pretty" as follows:
(define-syntax-rule (write-now proc x port mode)
(proc x port mode))
https://github.com/default-kramer/morsel/blob/master/morsel-lib/private/essence/from.rkt#L373
This is a test that fails in "incorrect but pretty" mode. Rackunit reports:
location: from.rkt:373:2
actual:
"(from #0=a/0 \"A\" (attach #1=b/1 \"B\" (attach #2=c/2 \"C\") #3=(list
#0# #1# #2#)))"
expected:
"(from a/0 \"A\" (attach b/1 \"B\" (attach c/2 \"C\") (list a/0 b/1
c/2)))"
Here are two screenshots, one pretty and one unpretty:
[image: pretty.png]
[image: unpretty.png]
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