Another thought: I think the best “feature” of transducers for performance-oriented language like Julia is actually not transducers themselves but rather the foldl implementations specialized for container types. I guess that’s not news since Julia Base has very efficient foldl/mapfoldl. However, foldl (or mapfoldl) itself is not really composable — that’s where transducers come in. Transducers are composable “pre-processors” of the “reducing function” op you passed to foldl. I think one of the great observations by Rich Hickey is that this set of pre-processors can be as powerful as the usual iterator tool chain. But, as @arghhhh pointed out, it requires a generalization of foldl for supporting early termination and completion. I think it is worth doing so since it can be compiled away if you don’t use it.
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