I wish to create a multi method which I call like this:
(defmethod some-method "some value" [ a b ] b) : but which selects the function based only on the first paramter 'a'. How can I do this:
(defmulti some-method WHAT GOES HERE?) 
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2 Answers
I didn't completely understand your question, but I think you want to dispatch only on one argument. You can do that like this, I think:
user=> (defmulti even-or-odd (fn [x _] (even? x))) #'user/even-or-odd user=> (defmethod even-or-odd true [a _] :even) #<MultiFn clojure.lang.MultiFn@293bdd36> user=> (defmethod even-or-odd false [a _] :odd) #<MultiFn clojure.lang.MultiFn@293bdd36> user=> (even-or-odd 2 3) :even user=> (even-or-odd 3 3) :odd user=> 
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Do you mean select the function based on the value of a?
Then you just need
(defmulti some-method (fn [a b] a)) 