Dennis Haupt <[email protected]> writes:
> i was taking a look at multimethods:
> (defmulti fac int)
> (defmethod fac 1 [_] 1)
> (defmethod fac :default [n] (*' n (fac (dec n))))
>
> this works
>
> however, this also works:
>
> (defmulti fac print)
> (defmethod fac 1 [_] 1)
> (defmethod fac :default [n] (*' n (fac (dec n))))
This definately does not work when I try it:
user> (defmulti fac print)
#<Var@6427854d: #<MultiFn clojure.lang.MultiFn@4bddef9a>>
nil
user> (defmethod fac 1 [_] 1)
#<MultiFn clojure.lang.MultiFn@4bddef9a>
nil
user> (fac 1)
IllegalArgumentException No method in multimethod 'fac' for dispatch
value: null clojure.lang.MultiFn.getFn (MultiFn.java:160)
1
The second arg to defmulti's job is to decide which method to call. The
print function always produces nil, so you would need a defmethod for
dispatch value nil to use print as a dispatch function:
user> (defmethod fac nil [_] 2)
#<MultiFn clojure.lang.MultiFn@4bddef9a>
nil
user> (fac 1)
12
nil
Notice the "12": the "1" is from print and the "2" is the value the
method produced.
Are you trying to print the dispatch values so you can see them for
tracing or something? If so, you could try something like:
user> (defn inspect [& stuff]
(println "inspect: " stuff)
(first stuff))
#<Var@5b1413a8:
#<user$eval336$inspect__337 user$eval336$inspect__337@68903261>>
nil
user> (inspect 1)
inspect: (1)
1
nil
user> (defmulti fac2 inspect)
nil
nil
user> (defmethod fac2 1 [_] 1)
#<MultiFn clojure.lang.MultiFn@e2df60d>
nil
user> (fac2 1)
inspect: (1)
1
nil
user>
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