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Simon St.Laurent wrote,
> This has come up before, but the chant "Postel's law has no
> exceptions" seems to be coming again, in the RSS context.
>
> http://tbray.org/ongoing/When/200x/2004/01/11/PostelPilgrim
>
> has Tim Bray's argument and lots of links to others.
I have to concede, even as a vigorous proponent of Postel's law, that I
find Tim's counter-example spectacularly clear, to the point, and hard
to rebut.
But what are we to conclude? That Postel's law it bunk? I don't think
so. Postel's law isn't a law of nature, or of logic. It's a law like
almost all the laws we have: a ceteris paribus law. But as is so often
the case, other thing _aren't_ always equal. As an engineering
principle, tho', it seems to hold enough of the time that it's
reasonable to take it as a default position. But that doesn't exclude
counter-examples; nor is the presence of counter-examples sufficient to
render the rule worthless.
IMO the right response would be to try and characterize the kinds of
circumstances in which it would be reasonable to expect the rule to be
reliable vs. those in which it wouldn't. I think that Tim's example is
quite amenable to that kind of characterization: it's one in which the
truncation of a message changes it's meaning in a significant way.
Whilst that's clearly a problem for the kind of message used in the
example, it's equally clearly not a problem in all cases. Many forms of
communicative act obey a kind of monotonicity principle ... more
information is always better, but that additional information only
allows the _refinement_ of a judgement, _not_ it's reversal. In those
kinds of case, a liberal stance towards truncation is reasonable,
always harmless, and typically beneficial.
Postel's law, crudely stated, is obviously too simplistic to be taken as
gospel. But as with all other ceteribus paribus laws it's possible to
treat it as a useful heuristic given an appropriate understanding of
the circumstances in which it's applied.
I'm not greedy ... that's good enough for me.
Cheers,
Miles
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