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Re: Postel's law, exceptions

• To: xml-dev@l...
• Subject: Re: Postel's law, exceptions
• From: Miles Sabin <miles@m...>
• Date: Tue, 13 Jan 2004 23:45:25 +0000
• In-reply-to: <r02000200-1028-D1176A3A460611D8AD220003937A08C2@[192.168.124.11]>
• References: <r02000200-1028-D1176A3A460611D8AD220003937A08C2@[192.168.124.11]>
• Sender: Miles Sabin <miles@m...>
• User-agent: KMail/1.5.4

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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    • From: "Simon St.Laurent" <simonstl@s...>

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