(The parentheses in the post title mean that this post is essentially parenthetical.)
Arggh. Developing Pixel Bender kernels and plug-ins, my test files kept behaving erratically. Expression indexes were to blame. Each time I added or removed a parameter, the numbers all shift.
So you see, first enabling the expression produces
effect("my_effect")(8). But it depends on the exact order of the parameters!
effect("my_effect")("param 9") depends on the name.
I think I like the second one better, because I’m pretty good about nailing down my parameter names, but more likely to move them around order-wise.