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Comment by GregBuchholz | Hacker News Reader

It looks like Racket nerfed the shared-structure / "Datum Labels" for circular lists and the like (I see it is still in section 2.4 of r7rs):

  '#0=(0 . #0#)

...and I should have included `#|block comments|#`, datum `#;comments`, and probably `|symbols with spaces|`.

  (let ((|1 + 2| (+ 1 2)))
    (display |1 + 2|))

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That has been in Racket for a long time.

I tried:

  #lang racket/base
  (let ((a '#0=(0 . #0#)))
    (display (car a)))

...over at

https://onecompiler.com/racket

...and it didn't compile, complaining:

  read-syntax: `#...=` forms not  enabled for `read-syntax` mode

...maybe there is a switch needed to enable it?

I am not 100% sure what's going on, but you can use `read` mode this way:

    #lang racket
    (let ((a (read (open-input-string "#0=(0 . #0#)"))))
      (display (car a)))

I think the problem is that `read` / `write` support shared data constructed using `shared`. But programs (read by `read-syntax` are not allow to have cycles.

Yeah, there's a toggle for it in read-syntax mode.

https://docs.racket-lang.org/reference/Reading.html#%28def._...

How do you get that to work?

  #lang racket
  (read-syntax-accept-graph #t)
  (let ((a '#0=(0 . #0#)))
    (display (car a)))

...which of course doesn't work, because `(read-syntax-accept-graph)` is a run-time thing, and the circular-list structure is a `read`-time thing.

I couldn't figure it out with this either:

https://stackoverflow.com/questions/51942188/read-syntax-for...

...just to make sure I wasn't going crazy, this:

  (let ((a '#0=(0 . #0#)))
    (display (car a)))

...does work as expected with this online scheme interpreter:

https://www.jdoodle.com/execute-scheme-online

It might take a custom #lang. I'll play around with it when I have some free time...

That read-syntax doesn't just accept everything accepted by plain read is one of my biggest annoyances with Racket.

It's ugly and probably fragile and prone to break, but I found a way. Added it as an answer on that Stack Overflow question: https://stackoverflow.com/a/79997503/9952196

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