This Week’s Program: Feb 29 - Mar 4

Last week I said:

Next week, I’ll be exploring more of the functionality of core.async to better handle song control flow…

I did that! I also switched AbstractFactory.tv’s SSL (or is it TLS?) cert away from Let’s Encrypt. I wrote about moving to Let’s Encrypt last December and now my 90-day cert was expiring. Let’s Encrypt makes automating all of this easy, but I couldn’t be bothered with that whole Docker thing again. In the past 90-days another major development in free SSL certificates emerged, AWS Certificate Manger. Since AbstractFactory.tv is hosted on AWS CloudFront, moving to AWS Certificate Manager was super easy. I still think Let’s Encrypt is a fantastic service, and free SSL/TLS certificates are something that is generally great.

Before getting into this week’s code, I want to highlight a commit from last week, after I wrote the TinyLetter. In order for my publication to TinyLetter to work, I had to patch a bit of code:

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GitHub has been pretty vocal about updating Jekyll for GitHub Pages, so I went ahead and made that bundle update last week. However, Jekyll’s internal API had changed quite a bit (which was expected, it was a major version release, after all). However, because my publishing to TinyLetter functionality is built as a Jekyll plugin, I had to do a bit of extra work to make that happen.

I was a bit dismayed by what I saw in Jekyll’s internals. The Jekyll::Document class is just a big mutable object. Local state is poison. In the latest Jekyll, the data described in the post’s front-matter is merged indiscriminately with the data from the rest of the site’s configuration and is no longer accessible on its own, which is a big deal for my POSSE-style generators. I have to be really explicit now when generating front-matter and that’s not ideal. I might submit a PR next week to Jekyll to have a way of accessing just the data captured in the post’s front-matter. I also might move this plugin strategy to be handled by a Jekyll hook. Whenever I decide to publish to Tumblr from Jekyll I’ll have to encounter this again.

Onward to this week.

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Before this commit, the drummachine fn would return a collection of async channels. The way I was reading from these channels was with the core.async/alts!! function. alts! and it’s sister function alts!! work like Go’s select: it awaits multiple channels until one of them arrives. That’s okay, but I really want to set up this operation so that the entirety of the drummachine completes at once. I want the drummachine API to appear like it is a single sequence.

To do that, I had to go a little deeper into core.async and I pulled out two useful functions.

The first is core.async/merge, which takes a collection of channels and linearizes their output into a single channel. Using this, when each instrument of the sequence completes, it will be an input into this composite signal.

The next is core.async/into. Like Clojure’s core into function, this is a pouring function: a collection of one type pours into the collection of the other. core.async/into returns a channel who’s value is all of the output of one channel poured into a collection.

The result is a single composite channel that blocks reading until all of it’s sub-channels complete, returning a vector of their output. When used in both the drummachine and the gen-song function, the API is vastly simplified.

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In this commit, I change the drummachine to use a multiple of the clock signal channel using core.async/mult. This makes it so that closing the master clock signal will close all of the components of the drummachine. I no longer need the step-sequencer function and later I change the name of play-sequence to sequencer.

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Finally, I use Clojure’s atom to synchronize all the active clock signals. This let’s me stop all or a portion of the actively playing sequencers. This is how you do global state.

Next week I’ll be focusing more on the music composition and theory parts of the program.

– Mark