idea, proposal: only preloadable libraries (conditional load)

Hello

I am searching way to extensible parser (actually not bison, only
transformations). I propose parserHook (transformation part). One
Tom's objection is difference between heooked and unhooked parser. It
serious problem.

I have one idea - only preloadable libraries. These libs have to be
specified in local_preload_libraries variable, and cannot be
initialised by LOAD statement. Statement LOAD should by used only for
reload library. Because pre loaded library is initialised before first
SQL statement should be executed, then hook is installed on time and
an behave is consistent. One technique is an calling library's
function "canload" before initialising. When canload returns false,
then dfmgr unloads lib. I thing so this behave complements current
functions PG_init and PG_finit. What I can understand, PG_init, cannot
throw exception or signalise any problems with initialisation.

Ideas, objections?

regards
Pavel Stehule