> Log Message:
> -----------
> Start to add support for speculative plugin loads.
>
> At present if libraries or symbols are missing,
> Felix throws an exception which terminates the program.
> Since catching exceptions is not really possible in Felix,
> I want to provide functions that return an error code
> instead of aborting.
>
> This might be used, for example, by the webserver,
> trying to load a formatting routine based on a filename
> extension. Plugins can then be written to support some
> formats and made available to the webserver whilst it is running.
Well, this turns out to be a bit harder than I thought.
With the current setup, it is possible find out if a library will
load, but not if it will initialise. One tests loading by simply loading it
then unloading it. If it is loadable .. you can load it again. Not very
pleasant.
However this doesn't check if a library will initialise.
Even if the initialisation symbols exist, they might not work.
In particular .. the library may load another one.
On the other hand, a library can do arbitrary work on initialisation,
in fact Felix programs are just library initialisations.
For plugins, there's an extra initialisation function
setup: string -> int
as well. In fdoc2html setup loads other libraries.
Now, if we do a speculative load for fdoc2html .. how do we
load the libraries which for it are optional? If we use a speculative
load for them, we may not get what we expect. After all then fdoc2html
will say it loaded or can be loaded. But actually it is not completely loaded.
So roughly speaking this is like const. It seemed like a good idea at the time
but it is a virus. It invades everything.
Exception handling isn't a good solution either. The problem is that,
assuming you could catch them .. well you have no idea what exceptions
might be thrown. An initialisation failure could be for many reasons, for
example a library may try to load a configuration file and be unable
to parse it.
And of course, Felix cannot (reliably) catch C++ exceptions, because they
unwind the machine stack, but Felix uses a heap allocated spaghetti stack.
On the other hand speculative loading is more or less essential.
One good example is a generic routine, hidden by a more specialised one.
You try for the specialised one and fall back on the generic one.
So you can get code working, and improve it later.
So I will have to carefully design what a speculative load actually does.
Probably it will return an error code if and only if the library doesn't
exist. Otherwise any failure throws an exception as usual. That still
doesn't provide any guidance for dependent libraries: basically the choice
of how to handle them would be up to the library using one of them
and not affected by how it itself was loaded (so the "speculative"
concept would not propagate).
--
john skaller
skal...@users.sourceforge.net
http://felix-lang.org
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