How to apply values to multiple functions in Clojure?

Question

Basically, I need to do something like map, but instead of applying a function to all elements in a collection, I need to apply the same (set of) value(s) to a collection of functions (does this operation have a name?). This might seem like a simple question, but I haven't found an idiomatic way to do it in Clojure. For the special case where I need to apply only one value to each function, for example, I have used

(for [f funs] (f value))

where value is, of course, the value I need each function to take as an argument, and funs is the collection of functions which need to be called with value as the argument.

My question is, then, is there a function in Clojure that does this, but is also generalised for arbitrary numbers of arguments? Or is the above indeed idiomatic Clojure?

sloth · Accepted Answer · 2013-07-26 07:30:10Z

23

You're looking for juxt

juxt

Takes a set of functions and returns a fn that is the juxtaposition of those fns. The returned fn takes a variable number of args, and returns a vector containing the result of applying each fn to the args (left-to-right). ((juxt a b c) x) => [(a x) (b x) (c x)]

answered Jul 26, 2013 at 7:30

sloth

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LawZero · Accepted Answer · 2016-07-23 05:24:05Z

9

From a section of CLOJURE for the BRAVE and TRUE

Another fun thing you can do with map is pass it a collection of functions. You could use this if you wanted to perform a set of calculations on different collections of numbers, like so:

(def sum #(reduce + %))
(def avg #(/ (sum %) (count %)))
(defn stats
  [numbers]
  (map #(% numbers) [sum count avg]))

(stats [3 4 10])
; => (17 3 17/3)

(stats [80 1 44 13 6])
; => (144 5 144/5)

answered Jul 23, 2016 at 5:24

LawZero

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2 Comments

monojohnny Over a year ago

It never occurred to me to put the "%" at the head of the list. Nice.

monojohnny Over a year ago

Great. This little example really helped me understand just that little bit more. Thanks! I see Clojure really is a first-class language. (that's a joke and an observation) (map #(apply % (range 1 3)) [+ - / *]); never occurred to me to put the functions in a list...

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