TL;DR

It's not too long, but, to summarize the summary, if you read Rich Hickey's 2014 blog post on transducers first, his 2012 post on reducers will be easier to understand.

Brief Definitions

Herewith, all in one place, are Clojuresque definitions of:

• reducible
• reducing function
• transducer
• reducer
• folder
• decomplected

Longer elaborations of these definitions follow in the subsequent section.

reducing function

Anything that can be used as the first argument of the reduce function, e.g. + or conj. Generally, it's a binary function that returns something of the type of its first argument, which is supposed to be a …

This is very cool.

(Thanks to David Nolen, who pointed out errors in the original.)

I hadn't realized that the standard higher order sequence functions compose in a manner that requires quite a bit of run-time machinery. For example nested map calls will cause a separate lazy sequence to be instantiated at each level of nesting, with each level "pulling" results from the next innermost as needed. The function calls are thus temporally interleaved, but the functions are not actually composed. We can see this happening by adding some printlns to some unary functions and then running a nested …