Ordering of versions

[Neil Bartlett]

On Mon, Nov 14, 2011 at 6:20 PM, cowwoc <cowwoc at bbs.darktech.org> wrote:
> On 14/11/2011 11:17 AM, Neil Bartlett [via jigsaw-dev] wrote:
> [snip]
>     Borrowing an analogy from Maven, you'd load the pom.xml file
> separately from the main artifact. The pom.xml provides you with the
> module metadata (version comparison in this case) and if you decide to
> proceed to fetch the rest of the module.

The analogy is bogus. Maven's POM -- like OSGi's manifest and Jigsaw's
module-info.java -- is a declarative XML format, not an executable. We
don't need to run any part of the module before we resolve the model,
we only need to read the metadata and execute the comparison which
exists inside the module system.

> 1) Treat the version comparator as a first-class citizen. There is one
> comparator across all versions of a module and developers may replace it
> as needed.

How do I replace the comparator when I find it has a bug? Presumably
until my new module with its new comparator can be installed, it must
be compared against the current installed module using the current
installed comparator. If the bug is as serious as, say, compareTo()
returning +1 irrespective of the arguments, then the current module
will always be considered higher than any replacement.

> 2) Each version has its own comparator. The first version that matches
> the desired version-range gets used. In my opinion, the ability to
> specify arbitrary version schemes is worth the risk of asymmetric
> results similar to Objects.equals().

Well I'm glad you see the risk, but I don't yet see the benefit. Is it
really useful for different modules to be able to redefine ordering of
versions? This is almost like each module wanting its own set of
natural numbers.

>     You could mandate that the Comparator operate deterministically,
> and then cache the results.

Please forgive the following diversion: a Russian friend once told me
this joke that was popular during the old Soviet days. Stalin,
Khrushchev and Brezhnev are riding a train together, and the train
breaks down. Stalin goes and shoots the driver and engineer, but this
doesn't solve the problem, the train doesn't move. Khrushchev offers
the (new) driver and engineer a nice flat in the suburbs of Moscow,
but this doesn't work either, and the train still doesn't move.
Brezhnev stands up and declares, "I will fix this problem". He closes
all the curtains in the train carriage, sits down and states "see, now
we are moving".

Mandating that a comparator must be deterministic is far less reliable
than using pure declarations and controlling the comparison within the
module system. Again, what's the benefit to balance this risk?

> Gili
>
>
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