Limit of form
Form is a useful concept but there is a limit.
Some types of composition simply do not have a structure. And, if they do, it is sometimes pretty difficult to discover what it is.
Generally, the longer a piece of music is, the more complex its structure. The nice and simple sectional form used in ABC notation is fine for shortish pieces of music but begins to feel the strain in longer pieces. More complex and hierarchical structures are called for in such music, sonata form being a good example.
Form is a positive hindrance in random music. If chance is your thing, then let luck determine where to go: toss a coin, roll dice, draw straws. Plenty of computer-generated noise is aleatoric in form, that is, based on chance. In fact, if you call your music aleatoric, nobody will understand you, or your music either, but they will be impressed.
Improvisation blurs the distinction between writing and performing music. Throughout the guide, writing and performing have been treated as related but distinct activities. There are advantages in treating writing music as a separate skill to performing music: it widens the field of music writers to include nonsingers and nonmusicians as well as singers and musicians; it emphasises the importance of timbre and the impact that instrumentation has on a piece of music; and it lessens the importance of key as transposition is done after a piece has been written. Improvisation makes the distinction less clear. Jazz improvisation is composing jazz on the spot. Live coding is instantaneous composition with a computer. In both cases, writing and performing are virtually instantaneous processes: as soon as a note or a sound is written it is played.
Improvisation is also at the limit of form. Some think improvisation has a form, that of theme and variation, in which the theme is written in advance and the variations improvised during live performance. Others argue that improvisation is intrinsically formless, and that, whatever it is, it is not composition.
Finally, improvisation is at the limit of writing music. There is absolutely nothing wrong with writing music for improvisation. Just scribble a theme, a melody or a chord progression will do, give it to your musician or computer friends and let them sort it out. Call it live coding, call it jazz, no one will complain.