It has been said that improvisation is composition but sped up. And vice versa. Either making decisions realtime or freezing (musical) time to consider the options before choosing one.
Maybe this is a fairy tale told to undergrads to get them over their fear of composition (not that their fear of improvisation is that much less) but it seems like an oversimplification.
When composing, you could certainly treat it like an improvisation, but invariably you'll want to alter some of the notes you've written. Or, you could make the future before you've written the present, giving yourself some direction.
And then there's the (pre)compositional process, which could entail an formal plan or simply deciding which algorithms to use. Improvisation often follows a form, even when that form is merely habitual like the incessant build in a Phish jam that leaves no listener uncertain where the climax is. But a general shape isn't a form—more akin to a narrative arc.
Sure there's a little overlap but very different processes, resulting in, generally very different styles of music.
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