[plt-scheme] A curious evaluation order (?) bug

From: Doug Williams (m.douglas.williams at gmail.com)
Date: Sun Dec 30 12:12:39 EST 2007

I sent an answer to Noel directly, but will post an answer for everyone
here, too.  I believe Noel expected the code sequence '(random-seed 1234)
(random-discrete discrete)' to always return the same value - because the
random source state should be the same.  Unfortunately, it isn't because the
science collection uses a different default random source than the PLT
Scheme one.

The science collection random source and distribution routines are built on
top of SRFI 27.  At the time the science collection was implemented, the PLT
Scheme random source routines were very naive and, therefore, the SRFI 27
implementation was used.  Because of restrictions within SRFI 27 (including
"Note that an assignment to default-random-source does not change random or
random-real; it is also strongly recommended not to assign a new value."), a
parameter, current-random-source, was created (in the science collection) to
mirror the current-pseudo-random-generator in PLT Scheme.

So, back to the '(random-seed 1234) (random-discrete discrete)' sequence.
The (random-seed 1234) call sets the seed in the PLT Scheme default random
stream (i.e., current-pseudo-random-generator).  The (random-discrete
discrete) call is using the current-random-source parameter (from the
science collection).  So, they are using completely different random

Another problem is the difference in philosophy between the PLT Scheme
random source interface and that of SRFI 27.  For 'seeding' a random source,
the PLT Scheme philosophy has a single random seed (set by random-seed) [and
maps directly to the original internal naive implenentation] and SRFI uses a
2-dimensional random-source-randomize! described as "Changes the state of
the random source s into the initial state of the (i, j)-th independent
random source, where i and j are non-negative integers. This procedure
provides a mechanism to obtain a large number of independent random sources
(usually all derived from the same backbone generator), indexed by two
integers. In contrast to random-source-randomize!, this procedure is
entirely deterministic."

In the 371 -> 372 transition (I believe at 371.1), the PLT Scheme random
source routine was changed to use the same algorithm as SRFI 27 and SRFI 27
itself was reimplemented in terms of the new PLT Scheme implementation.
There was some breakage to the science collection during that transition,
but I think it's all been fixed.  [You can look at the change log on PLaneT
for details.]  However, the random sources are different struct types
internally and cannot be used interchangeably.  So, we still have the same
general problem of two different random source implementations - the science
collection happens to use and extend the SRFI 27 one.

I'm not sure of the best way to eliminate that confusion in the long term
(other than through documentation), but am open to suggestions.


On Dec 29, 2007 9:26 AM, Noel Welsh <noelwelsh at gmail.com> wrote:

> Here's something curious.  I have code that samples from a discrete
> distribution, and it does not behave as I would expect.  The code is
> thus:
> (define (sample-cluster-mixture state discrete)
>  (define clusters (state-clusters state))
>  (define i (random-discrete discrete))
>  (printf "SAMPLE CLUSTER MIXTURE ~a\n" i)
>  (printf "~a ~a\n" (discrete-pdf discrete 0)
>          (discrete-pdf discrete 1))
>  (random-seed 1234)
>  ;(display (random-discrete discrete))
>  (if (zero? i)
>      (sample-new-cluster state)
>      (vector-ref clusters i)))
> The important thing here is the value of i, which is printed out.
> I call (random-seed 1234) and run it, and the output is this:
> 0.41176470588235303 0.588235294117647
> 0
> So i is 1
> I uncomment the display line and run it, and get:
> 0.41176470588235303 0.588235294117647
> 0
> So now i is 0
> Note that i is evaluated before the call to display.  All I can think
> it that this is an optimiser bug, reordering expressions in some
> illegal way.  I'm using  v371.4 [3m].  I've just svn up'ed and I'll
> try v4 and 372 to see if there is any difference.  If not I'll try to
> narrow the bug down.
> Consider this a preliminary report only.  I was so surprised by this
> (having spent an few hours debugging) I had to let the world know...
> N.
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