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)
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