Deferred initialization of global variables

[shirok]

A mechanism to compute its value when it is first referenced, and store the result so that the subsequent reference returns the computed value.

It can be done with gauche.threads#run-once:

(use gauche.threads)
(define *deferred* (run-once (compute-the-value))

However, there's another use case - to compute a value that depends on the runtime environment but known to be constant during the run (#1001). Such value is idempotent and run-once is overkill. We also want subsequent access to be as lightweight as possible,

Global varaible with a special getter hook may be good.