Pattern matching functions in Clojure?

Question

I have used erlang in the past and it has some really useful things like pattern matching functions or "function guards". Example from erlang docs is:

fact(N) when N>0 ->      N * fact(N-1);  fact(0) ->           1.     

But this could be expanded to a much more complex example where the form of parameter and values inside it are matched.

Is there anything similar in clojure?

Answer 1

There is ongoing work towards doing this with unification in the core.match ( https://github.com/clojure/core.match ) library.

Depending on exactly what you want to do, another common way is to use defmulti/defmethod to dispatch on arbitrary functions. See http://clojuredocs.org/clojure_core/clojure.core/defmulti (at the bottom of that page is the factorial example)

Answer 2

I want to introduce defun, it's a macro to define functions with pattern matching just like erlang,it's based on core.match. The above fact function can be wrote into:

(use 'defun) (defun fact   ([0] 1)   ([(n :guard #(> % 0))]      (* n (fact (dec n))))) 

Another example, an accumulator from zero to positive number n:

(defun accum   ([0 ret] ret)   ([n ret] (recur (dec n) (+ n ret)))   ([n] (recur n 0))) 

More information please see https://github.com/killme2008/defun