The short answer is that you're right: creating new values at custom-print time creates trouble for graph detection and `print` quoting. Those operations perform a pass to make decisions about graphs and quoting based on `eq?` identity, and then they make another pass to actually print. I'll update the documentation to clarify.
A longer answer is that your example and variations expose several mismatches between the `racket/pretty` printer, the built-in printer for current Racket, and the built-in printer for Racket CS. I'm working on changes and tests to make them more consistent, but the changes don't produce the result you wanted. If you put `maybe-rebuild` in `custom-print`, then they more consistently produce the output that you don't want. Of course, an even better improvement would collapse the three different implementations of the printer. One day, we may be able to use the Racket CS I/O layer in place of the implementation in current Racket, and then folding in pretty-print support may become practical. At Fri, 10 Jan 2020 16:58:34 -0800 (PST), Ryan Kramer wrote: > I have a small program that demonstrates some surprising behavior regarding > `prop:custom-print-quotable 'never`. Perhaps it could be considered a bug, > although I haven't seen anything in the docs that this violates. I've found > that > > 1. When I call `(println y)`, my `custom-display` is called first. And what > I do in `custom-display` affects the result of `(println y)` > 2. The printer is doing something with `eq?` rather than `equal?`. (Does > this mean that constructing any new data structure, even a list, during > printing is an anti-pattern?) > > In the example below, if `(maybe-rebuild lst)` returns `(identity lst)` > then `prop:custom-print-quotable 'never` works as I expected. But if > `(maybe-rebuild lst)` returns `(apply list lst)` which is a list that is > equal? but not eq? to the original list, then `prop:custom-print-quotable > 'never` surprises me. > > #lang racket > > (require racket/struct) > > (struct my-struct (item) #:transparent > #:property prop:custom-print-quotable 'never) > > (define (go port mode val) > (let ([proc (make-constructor-style-printer > (λ (me) 'my-class%) > (λ (me) (list val)))]) > (proc (void) port mode))) > > (define (maybe-rebuild lst) > ; Works as expected if we return the same list > #;(identity lst) > ; But this breaks prop:custom-print-quotable 'never > (apply list lst)) > > (define my-class% > (class* object% (printable<%>) > (super-new) > (init-field content) > (define/public (custom-print port mode) > (go port mode content)) > (define/public (custom-write port) > (go port #t content)) > (define/public (custom-display port) > (go port #f (maybe-rebuild content))))) > > (define x (list 'CONTENT (my-struct '(1 a)))) > (println x) > (define y (new my-class% [content x])) > (println y) -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/20200114203624.76CA8650189%40mail-svr1.cs.utah.edu.