Is there a reducing function in Clojure that performs the equivalent of `first`?

Question

I'm often writing code of the form

(->> init
     (map ...)
     (filter ...)
     (first))

When converting this into code that uses transducers I'll end up with something like

(transduce (comp (map ...) (filter ...)) (completing #(reduced %2)) nil init)

Writing (completing #(reduced %2)) instead of first doesn't sit well with me at all. It needlessly obscures a very straightforward task. Is there a more idiomatic way of performing this task?

Answer 1

I'd personally use your approach with a custom reducing function but here are some alternatives:

(let [[x] (into [] (comp (map inc) (filter even?) (take 1)) [0 1])]
    x)

Using destructing :/

Or:

(first (eduction (map inc) (filter even?) [0 1])

Here you save on calling comp which is done for you. Though it's not super lazy. It'll realize up to 32 elements so it's potentially wasteful. Fixed with a (take 1):

(first (eduction (map inc) (filter even?) (take 1) [0 1]))

Overall a bit shorter and not too unclear compared to:

(transduce (comp (map inc) (filter even?) (take 1)) (completing #(reduced %2)) nil [0 1])

If you need this a bunch, then I'd probably NOT create a custom reducer function but instead a function similar to transduce that takes xform, coll as the argument and returns the first value. It's clearer what it does and you can give it a nice docstring. If you want to save on calling comp you can also make it similar to eduction:

(defn single-xf
  "Returns the first item of transducing the xforms over collection"
  {:arglists '([xform* coll])}
  [& xforms]
  (transduce (apply comp (butlast xforms)) (completing #(reduced %2)) nil (last xforms)))

Example:

(single-xf (map inc) (filter even?) [0 1])

Answer 2

medley has find-first with a transducer arity and xforms has a reducing function called last. I think that the combination of the two is what you're after.

(ns foo.bar
  (:require
   [medley.core :as medley]
   [net.cgrand.xforms.rfs :as rfs]))

(transduce (comp (map ,,,) (medley/find-first ,,,)) rfs/last init)