Clojure中的模式匹配函数？

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

Answer 3

core.match is a full-featured and extensible pattern matching library for Clojure. With a little macro magic and you can probably get a pretty close approximation to what you're looking for.

Answer 4

Also, if you want to take apart only simple structures like vectors and maps (any thing that is sequence or map, eg record, in fact), you could also use destructuring bind . This is the weaker form of pattern matching, but still is very useful. Despite it is described in let section there, it can be used in many contexts, including function definitions.