hckrnws

A popular noob error in Clojure is to get really, really excited about using reduce and trying to use it to replace for loops. Maybe someone wants to sum up every other odd number and they build some horrible reducer to do that which manages its own state in the reduction object. It leads to ugly code. Reduce and it's colleague map are most powerful when the underlying collection could have been be shuffled and it wouldn't matter much - or at least where the state to be managed in the reduce is trivial.

So something to note about transduce is that it looks a lot like reduce but suddenly the examples [0] start being more complex functions where some elements of the collection get removed, order matters and state begins to appear during the fold. Functions like `take`, `filter`, `partition`, `interpose`, `dedupe`, etc, etc.

As with many things in the functional world (cough monads) I feel the explanations don't make a lot of sense before meeting the problem they solve in the wild. As with a lot of Clojure's better parts transducers are a state management tool.

[0] https://clojure.org/reference/transducers

This is maybe as good a time as any to discuss the usual alternative presentation of transducers that I find is a far gentler introduction.

A transducer is simply a function of the form `a -> List<b>`. Yes this is fully polymorphic, i.e. it holds even for transducers that only operate over just lists.

In this case `mapping` is just

This lens views a transducer through the lens of the `eduction` function as a more souped up `map`. Whereas with a normal `map` you only get to use a function `a -> b`, which means you can't change the underlying structure, with `eduction` you get to use `a -> List<b>` (again a transducer), where you "keep" any elements that are in the output list and you "discard" any elements not in the output list.

So e.g. `mapping` would be:

    (defn mapping [f] (fn [x] [(f x)]))

    def mapping(f):
        return lambda x: [f(x)]

    (defn filtering [f] (fn [x] [if (f x) [x] []]))

    def filtering(f):
        return lambda x: [x] if f(x) else []

For more details on the full equivalence see https://news.ycombinator.com/item?id=27778423

The current presentation of transducers has always felt like a wart in Clojure.

Nicely done.