Hello,

I was wondering if it isn't worth to include the timing part
of SRFI-18 in the small version of Scheme. The procedures
that I suppose would be interesting are:

- (current-time)
- (time->seconds time)
- (seconds->time x)
- (time? obj)

Just that. No threads, mutexes or signals.

The reason for this is that those procedures would be useful
to implement two important didactic examples:

- A PRNG:
  I agree with those that think that including simple randomness
  in "small" Scheme is not encessary, since it's easy to write a
  PRNG that would work fine for didatic purposes: a linear
  congruential PRNG is trivial to write, easy to understand and
  a nice example of procedure. It's also a nice example of a
  referentially transparent procedure if you don't hide its
  state -- and later in the course it's possible to talk about
  encapsulation of state using it again as an example.
  However, it would be good to have some way to seed the PRNG
  with a minimally unpredictable number, which could be obtained
  from (time->seconds (current-time))  (for simple randomness
  for students this is usually good enough).

- A threading system:
  A didactic Metacircular evaluator could in principle
  be used to teach about threading: it would switch threads
  after K calls to eval. But having (current-time) available
  would allow one to write a metacircular evaluator that
  actually checks, at the end of EVAL, if a certain quantum
  of time has passed -- and then switch threads if so.
  This is conceptually closer to a threading system with
  preemption and would make a better example (although I also
  think coroutines should be taught -- and I already do that).

What do others think?

-- Jeronimo Pellegrini

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