Hi there,

I've been playing around with Guile's implementation of SRFI 45 (which
uses SRFI 9), and have noticed something interesting: if you run the
following code in the REPL, printing out the value of the promise will
cause a stack overflow:

    (use-modules (srfi srfi-45))
    (define promise (delay promise))
    (force promise)

    #<promise val: module/ice-9/format.scm:38:0: In procedure format:
    module/ice-9/format.scm:38:0: Throw to key `vm-error' with args `(vm-run "VM: Stack overflow" ())'.

However, if I change the SRFI 45 code to use Guile's native records,
it prints correctly:

    #<promise val: #<value tag: eager proc: #-1#>>

For reproducibility, I attached the diff to make SRFI 45 use Guile
native records.

So, it seems like SRFI 9's record printer doesn't handle cycles in the
record data very well. Unfortunately I don't know how to fix this (I
tried looking at the printer for Guile's native records, but couldn't
figure out what made it special enough to handle cycles correctly), so
no fix is attached.

Many thanks,
Chris.