Why custom-display of printable<%> interface is called twice?

[Jesse Wang]

Hi, I created a simple class to test the printable<%> interface:

(define my-class%

  (class* object% (printable<%>)

    (super-new)

    (define/public (custom-display out)

      (displayln "custom-display"))

    (define/public (custom-write out)

      (writeln "custom-write"))

    (define/public (custom-print out depth)

      (print "custom-print\n" out))))

But when I try to display an object of this class, the custom-display function is called twice, why?

>(define obj (new my-class%))

>(display obj)

custom-display

custom-display

And When I write the object, custom-display is also called first:

> (write obj)

custom-display

"custom-write"

With print:

> (print obj)

custom-display

custom-display

"custom-print\n"

I think I must have missed something here, and I can't find anything about this in the documentation of printable<%>, please help.

Thanks.

[Sorawee Porncharoenwase]

I’m not sure why it’s like that, but I know what’s the problem and how to fix it.

custom-display and custom-write provide the output port to you. You are supposed to write to the port.

    (define/public (custom-display out)
      (displayln "custom-display" out))
    (define/public (custom-write out)
      (writeln "custom-write" out))

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[Jesse Wang]

Thanks, it works!

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[Philip McGrath]

Someone had a very similar question about `printable<%>`, including both the confusion about multiple calls and the bug of failing to use the `out` port, on Stack Overflow recently: https://stackoverflow.com/questions/55975708/racket-equivalents-of-pythons-repr-and-str/56759937#56759937

Since these have apparently been points of confusion for multiple people, I'm wondering if there's something that could be improved in the documentation to help.

-Philip

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