Very cool to see more projects in this space. Personally, I don't like the "bow-tie" operator. It's unprecedented and doesn't convey the sense of direction. Why not utilize the often-used `|>` (as seen in OCaml, F#, JS TC39, Elm, Elixir) or just reuse zero_functional `-->`?
For me, contrary to @elritch's experience, reusing `[]` for declaring idents looks poorly readable. Overloading the meaning of a basic language construct is adding to the mental overhead. I don't mind the slightly longer pseudo-closure syntax, as it's unambiguous and non-intrusive. I also see the ability to define custom consumers (which you call "reducers"), but IMO it's only useful if the choice of iterator adapters (aka transformers) is adequate. What's the status here? Are there any plans to provide some DSL or other way for defining custom adapters? As the final idea, some comparisons of the resulting macro expansion between iterrr and zero_functional would be very interesting to see.
