Chapter 30. A Tour of the Song Roster

Goal

By the end of this chapter you will have heard the full range of what a Keleusma song can do, and understood why that range is so wide.

A song is a program

The reason the roster is worth a tour is a single fact established across the whole guide. A song is not a list of notes. A song is a program. It can compute, branch, count, and change its own behaviour over time, because it is built from functions, match, the data segment, and a loop. Anything a bounded program can do, a song can do. The ten bundled songs are chosen to show that range.

Run the piano roll from Chapter 28 and press s, or a number key, to move through them.

The roster

• Songs 0 and 1 are the introductory pieces. Three channels, a four-bar chord progression. They are the ones to read first, and the ones Chapter 29 modified.
• Song 2 is an arrangement of a Bach prelude across five channels. It shows the host’s envelope and retrigger handling on real repertoire.
• Song 3 is an eight-channel piece in D minor that exercises every host native, shifts into a seven-beat meter partway through, and ramps its tempo continuously.
• Song 4 runs its tempo under a slow sine wave, between 60 and 300 beats per minute, and varies its material every time its loop counter advances, in the manner of an algorithmic variation form.
• Song 5 is a process piece. Eight voices play the same twelve-note pattern, but each advances at a different rate, so the voices drift into and out of alignment over minutes and hours.
• Song 6 is a canon whose four voices share one melody but move at four different speeds at once, producing genuine four-voice counterpoint.
• Song 7 is a microtonal drone. Its eight voices are tuned to the natural harmonic series, using fine pitch offsets the host supports directly.
• Song 8 is a conventional pop song, with a bridge and a key change. It is here to show that the same engine that runs the experimental pieces also handles ordinary songwriting.
• Song 9 is a long experimental loop that cycles through sixteen variations of scale and instrument, running about fifty minutes before it repeats.

Why this matters

Several of these songs do things a conventional music program, a digital audio workstation, would struggle with. A tempo shaped by a continuous sine wave, four meters running at once, a tuning drawn from the harmonic series, a piece that restructures itself on a counter: these are not points on a menu. They are consequences of the song being a program.

That is the demonstration. The experimental songs are not there because they are pleasant. They are there because they could not easily exist any other way, and the fact that they run, within the same proved time and memory bounds as song 0, is the language showing what it is for.

What you now know

• Every song in the roster is a program, which is why the roster ranges so widely.
• The bundled songs span introductory progressions, a Bach arrangement, several experimental process and tuning pieces, and a conventional pop song.
• Techniques that are awkward or impossible in a conventional music program follow naturally when the song is a program.

That completes Part VIII, and with it the part of the guide written for someone learning the language. Part IX is a separate track, for a developer who wants to host Keleusma inside a Rust program of their own.