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How do I create a recursive anonymous function in Clojure which is not tail recursive?

The following clearly doesn't work, as recur is only for tail recursive functions. I'm also reluctant to drag in a y-combinator..

((fn [n] (if (= 1 n) 1 (* n (recur (dec n))))) 5)
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  • "Just give it a name"? They call me Mr. Anonymous... :P~ Commented Apr 11, 2011 at 19:49

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Functions can be given a name to refer to themselves by specifying it between fn and the arglist:

user> ((fn ! [n] (if (= 1 n) 1 (* n (! (dec n))))) 5)
120
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3 Comments

yay for naming anonymous functions! +1
Calling it ! seems to run counter to convention, since it's not mutating anything.
The function he's defined is the factorial function. I could have called it fact, but mathematicians call it !, and we're working in a language that allows us to call it the same. Seems silly to write something longer when everyone knows that 5! is 120.
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Here's a way that keeps it anonymous, mostly:

(((fn [!] (fn [n] (if (= 1 n) 1 (* n ((! !) (dec n)))))) 
  (fn [!] (fn [n] (if (= 1 n) 1 (* n ((! !) (dec n))))))) 
 5)

It's not quite the Y combinator, but it does contain the same bit of self-application that allows Y to do its thing. By having a copy of the entire function in scope as ! whenever you need it, you can always make another copy.

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You can remove some of the duplication there with this: (((fn [!] (fn [n] ((! !) n))) (fn [!] (fn [n] (if (= 1 n) 1 (* n ((! !) (dec n))))))) 5) This clearly isn't the way to do it (given that Clojure can name the fns, but I still like it. :)

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