composing functions as optional arguments to higher-level function in clojure

Question

I'm trying to create a multiple-arity function in Clojure that, if given a single vector argument, just applies another function to the vector; and if given a vector plus 1 or more functions, maps the composition of those functions over the vector then applies the third function. But somehow I don't seem to be able to access any optional argument outside the first.

Toy example code:

(defn times2 [num] (* 2 num))
(defn square [num] (* num num))
(defn plusfiveall [vec-of-nums] (map #(+ 5 %) vec-of-nums))
(defn toynums
  ([vec-of-nums] 
   (plusfiveall vec-of-nums))
  ([vec-of-nums & [funcs]]
   (let [moremath (comp funcs)]
      (plusfiveall (map moremath vec-of-nums)))))

What I think should happen:

(toynums [1 2 3]) = (6 7 8)

(toynums [1 2 3] times2) = (7 9 11)

(toynums [1 2 3] times2 square) = (7 13 23)

but while the first two of those examples work as expected, the third fails to apply square: instead I get (toynums [1 2 3] times2 square) --> (7 9 11)

In an attempt to drill down on the problem, I've also tried a version without the mapping, and have the same surprising behavior:

(defn comp-nomap 
  ([num] (plusfive num))
  ([num & [funcs]] 
   (let [funmath (comp funcs)] 
     (plusfive (funmath num)))))

(comp-nomap 3) = 8 as expected

(comp-nomap 3 times2) = 11 as expected

but (comp-nomap 3 times2 square) also = 11, rather than 23 like it should be.

help? thanks!

Piotrek Bzdyl · Accepted Answer · 2016-03-05 18:04:46Z

2

Your second arity uses destructuring of the varargs which will always extract the first function from the supplied ones. You also need to apply comp function to arguments in a sequence:

(defn comp-nomap
  ([num] (plusfive num))
  ([num & funcs]
    (let [funmath (apply comp funcs)]
      (plusfive (funmath num)))))

answered Mar 5, 2016 at 18:04

Piotrek Bzdyl

13.2k1 gold badge36 silver badges51 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Paul Gowder Over a year ago

thinking out loud to make sure I get it... my original version destructures the remaining & arguments on the way into the function? so when I tried (comp-nomap 3 times2 square) it saw (times2 square) as the & arguments, pulled out the first of those and assigned it to func, discarding the rest... is that right?

Chris Murphy · Accepted Answer · 2016-03-05 18:18:58Z

2

This construction produces the expected answers:

(defn toynums
  ([vec-of-nums]
   (plusfiveall vec-of-nums))
  ([vec-of-nums & funcs]
   (let [moremath (apply comp funcs)]
     (plusfiveall (map moremath vec-of-nums)))))

The arguments after the & are automatically collected together into a sequence, so that inside you can use apply, which allows the comp function to have the contents of the sequence passed to it as arguments. So really you had it correct, just missed that one thing. It is 'varadic' arguments you need to look up.

edited Mar 5, 2016 at 18:18

answered Mar 5, 2016 at 17:59

Chris Murphy

6,5092 gold badges27 silver badges46 bronze badges

1 Comment

Paul Gowder Over a year ago

thank you! the same strategy works with my even smaller toy example too. Can you/anyone explain why? I'm starting to suspect that the answer involves the mysteries of destructuring, and I don't understand it at all...

Collectives™ on Stack Overflow

composing functions as optional arguments to higher-level function in clojure

2 Answers 2

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related