Question

I’m new to clojure, and I’ve seen anonymous functions written like:

(fn [x] (* x x))

and also like:

#(* % %)

Obviously, the second is more concise. Is there any relevant difference? Can every anonymous function be represented in either style? Is one more idiomatic?

Related to this question, I was unable to determine how to convert (fn [x] [x x]) to the latter syntax. I would appreciate a pointer to documentation that clarifies this situation.

Answer 1

The most important differences are:

• (fn ...) can be nested, #() cannot
• You can name your parameters better with (fn [x y] ..) or similar, rather than using %, %2, %3 etc.
• You can name a function with (fn ...) for recursive usage, e.g. (fn fib [n] (if (<= n 1) 1 (+ (fib (- n 1)) (fib (- n 2)))))
• It’s easier to do code generation/manipulation (fn [...] ...) since #() is a reader macro rather than a regular Clojure form.
• #() is more concise. But if that is a major consideration, you probably have your priorities wrong 🙂

Personally my advice would be:

• Prefer (fn [...] ...) in most circumstances
• Use #() only for very short inline functions, e.g. (map #(+ 2 %) (range 10))
• Also consider that it may be better to generate anonymous functions via higher-order functions rather than writing them out explicitly e.g. (comp func1 func2) or (partial func param1 param2) etc.