Am Do., 27. Aug. 2020 um 04:14 Uhr schrieb Alex Shinn <xxxxxx@gmail.com>:

The current state is that, per R7RS, any cyclic literals must be directly quoted,

which means match-letrec can support them.  The idea of cycles in indirectly

quoted data, via other macros, is disallowed by the standard, with quasiquote

given as a specific example.  So I guess we can leave the cycles as is.

(let-syntax

    ((q

      (syntax-rules ()

        ((_ x) (quote x)))))

  (q #0=(#0#)))

is allowed by my reading of R7RS and is logically sound. Circular references are allowed only in literals but whether a certain datum in the input is a literal can only be detected after macro expansion. The example of quasiquote doesn't apply because quasiquote does not need to expand into quote.