[Python-Dev] PEP 246, redux

[Alex Martelli]

On 2005 Jan 11, at 20:44, Phillip J. Eby wrote:
    ...
>> If we're just recomending that people design for transitivity, then I
>> don't have a problem (although see Alex's fairly good point 
>> illustrated
>> with LotsOfInfo, PersonName, and FullName -- I found it convincing).
>
> It's a bit misleading, however; if the target protocol allows for 
> "nulls", then it's allowed to have nulls.  If it doesn't allow nulls, 
> then the adaptation is broken.  Either way, it seems to me to work 
> out, you just have to decide which way you want it.

NULLs are allowed, but *PRAGMATICALLY* they shouldn't be used except 
where there's no alternative.

Is the concept of *PRAGMATICS* so deucedly HARD for all of your 
eggheads?!  Maybe a full-credits course in linguistics should be 
mandatory for CS major or wherever you got your sheepskin[s].

In terms of syntax and semantics, a TCP/IP stack which just dropped all 
packet instantly would be compliant with the standards.  No GUARANTEE 
that any given packet will be delivered is ever written down anywhere, 
after all.

The reason such a TCP/IP stack would NOT be a SENSIBLE implementation 
of the standards is PRAGMATICS.  The stack is supposed to do a 
best-effort ATTEMPT to deliver packets, dammit!  That may be hard to 
formalize mathematically, but it makes all the difference in the world 
between a silly joke and a real-world tool.


My best example of pragmatics in linguistics: if I state...:

""" I never strangle python-dev posters with the initials PJE in months 
with an "R" in them """

I am saying nothing that is false or incorrect or misleading, in terms 
of syntax and semantics.  This assertion is grammatically correct and 
semantically true.

Does this mean you should worry come May...?  Not necessarily, because 
the assertion is _pragmatically_ dubious.  *PRAGMATICALLY*, in all 
natural languages, when I state "I never do X under condition Y" 
there's an implication that "condition Y" DOES have something to do 
with the case -- that if condition Y DOESN'T hold then my assurance 
about not doing X weakens.  If condition Y has nothing to do with my 
doing or not doing X, then by the PRAGMATICS of natural language I'm 
NOT supposed to juxtapose the two things -- even though both 
syntactically and semantically it's perfectly correct to do so.

Network protocol specs, programming language, libraries, etc, have 
pragmatics, too.  They're way harder to formalize, but that doesn't 
mean they can be blithely ignored in the real world.


Yes, you're ALLOWED to stuff with NULL any field that isn't explicitly 
specified as NOT NULL.

But you should ONLY do so when the information is REALLY missing, NOT 
when you've lost it along the way because you've implemented 
adapter-chain transitivity: dropping information which you COULD have 
preserved with a bit more care (==without transitivity) is a violation 
of PRAGMATICS, of the BEST-EFFORT implication, just as it would be to 
drop packets once in a while in a TCP/IP stack due to some silly 
programming bug which was passed silently.


Alex