Example
There are two ways to define an anonymous function: the full syntax and a shorthand.
Full Anonymous Function Syntax
(fn [x y] (+ x y))
This expression evaluates to a function. Any syntax you can use with a function defined with defn (&, argument destructuring, etc.), you can also do with with the fn form. defn is actually a macro that just does (def (fn ...)).
Shorthand Anonymous Function Syntax
#(+ %1 %2)
This is the shorthand notation. Using the shorthand notation, you don't have to name arguments explicitly; they'll be assigned the names %1, %2, %3 and so on according to the order they're passed in. If the function only has one argument, its argument is called just %.
When To Use Each
The shorthand notation has some limitations. You can't destructure an argument, and you can't nest shorthand anonymous functions. The following code throws an error:
(def f #(map #(+ %1 2) %1))
Supported Syntax
You can use varargs with shorthand anonymous functions. This is completely legal:
#(every? even? %&)
It takes a variable number of arguments and returns true if every one of them is even:
(#(every? even? %&) 2 4 6 8)
;; true
(#(every? even? %&) 1 2 4 6)
;; false
Despite the apparent contradiction, it is possible to write a named anonymous function by including a name, as in the following example. This is especially useful if the function needs to call itself but also in stack traces.
(fn addition [& addends] (apply + addends))
