what does := means simply?

[Peter J. Holzer]

On 2018-05-20 16:36:12 -0400, Richard Damon wrote:
> 2) Try to maximize portability by not only looking at the specs, but
> also common implementations, and choosing the options that maximize the
> acceptability of your output to tools that don't fully meet the specs.
> Also, if a common implementation generates something not quite to the
> standard, try to make it so you can accept that output too.

This is the well-known "be conservative in what you send and liberal in
what you accept" principle. 

It has fallen into disfavour over the last decade or so. There are
several reasons:

* Being liberal in what you accept is problematic because you are
  accepting input which has no specified meaning and interpret it as you
  see fit - but there is no guarantee that this is the interpretation
  that the sender intended. This may result in silent data loss. In many
  cases an error message is better.
* Accepting non-standard input is also problematic because such input is
  probably not well-tested. The code is much more likely to contain
  bugs, maybe even security-critical bugs.
* If some features of a spec are rarely used, programmers may not
  implement them. When they are needed, they won't work. A recent
  example is that the TLS working group found that they can't use the
  version number field to signal the version number because too many
  implementations got it wrong (I have no idea how that happened. We are
  already on the 6th version and all previous upgrades used the version
  field).

Of course, if a popular implementation has known bugs you may have no
choice but make concessions.

        hp


-- 
   _  | Peter J. Holzer    | we build much bigger, better disasters now
|_|_) |                    | because we have much more sophisticated
| |   | hjp at hjp.at         | management tools.
__/   | http://www.hjp.at/ | -- Ross Anderson <https://www.edge.org/>
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