[racket] what is this thing: '#(struct:foo 3 4) ?

I've spent long enough with the docs on this not to feel too embarrassed about this question: what kind of value is produced by

'#(struct:foo 3 4)

(N.B. the leading quote)

... and how can I extract its values?

The problem is that I'm trying to use "write" and "read" on the built-in "date" structure. The problem is that since date is #:transparent but not #:prefab, reading it back in doesn't produce a date structure, but rather one of these quoted thingies.  I'm guessing there's a simple way to, e.g., transform it into a list, but (having been through the docs for 'write' and for structures), I'm not sure where to find  out what it is.

John
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