Why should I use 'apply' in Clojure?

Question

This is what Rich Hickey said in one of the blog posts but I don\'t understand the motivation in using apply. Please help.

A big difference between Clojur

Answer 1

You use apply to convert a function that works on several arguments to one that works on a single sequence of arguments. You can also insert arguments before the sequence. For example, map can work on several sequences. This example (from ClojureDocs) uses map to transpose a matrix.

user=> (apply map vector [[:a :b] [:c :d]])
([:a :c] [:b :d])

The one inserted argument here is vector. So the apply expands to

user=> (map vector [:a :b] [:c :d])

Cute!

PS To return a vector of vectors instead of a sequence of vectors, wrap the whole thing in vec:

user=> (vec (apply map vector [[:a :b] [:c :d]]))

While we're here, vec could be defined as (partial apply vector), though it isn't.

Concerning Lisp-1 and Lisp-2: the 1 and 2 indicate the number of things a name can denote in a given context. In a Lisp-2, you can have two different things (a function and a variable) with the same name. So, wherever either might be valid, you need to decorate your program with something to indicate which you mean. Thankfully, Clojure (or Scheme ...) allows a name to denote just one thing, so no such decorations are necessary.