A little of both. When I was designing the instrument, I definately took the "Let's see how this sounds?" approach. I did a lot of experimenting generating csound code with perl. After my experiments, I had a bunch of material that was ready to be made into the piece. Rain was part of this material.

When I was ready to compose Rain, the instrument had already been developed. I took my Rain instrument and made it a perl sound object (Rain.pm).

But as for composition goes, it was very much a "That's what I want to hear" type thing. I spent a long time fine tuning the sections of the piece and rewritting code to get it do pseudo-exactly what I wanted to do. I say pseudo-exactly because it doesn't generate the exact same csound score everytime. But it always generates within the limits of where I want everything to be.

I should also point out that the Rain sound is based on something I call a Harmonic Tree.

The whole thing took 9 months to complete. But to be fair, I wasn't working on it the entire time.