问题
As also explained in the answers of this question, Clojure determines the number of parameters of an anonymous function (defined through #()), by the maximal parameter index referenced in the body (e.g. if the maximal referenced parameter is %4, then that anonymous function has 4 parameters).
Question: is there some syntax to tell the Clojure compiler that an anonymous function expects one parameter, even not referencing that parameter? Or, in this case, the only "clean way"* is to use the fn syntax? (* "Clean way": I'm aware that in most cases, you could reference the dummy parameter without affecting the function -- see below, but I would like to avoid such workarounds.)
My use case: I have a function (defn foo [predicate]), where predicate expects exactly one argument, and I would like to test foo with a predicate that always returns true. (I.e., something like #(true).) This causes an ArityException since the interpreter thinks that #(true) expects zero arguments.
In this particular case, I could use some clever hack like #(or true %), but:
- It is almost as much to type as the
fn-variant. - I'm interested if there is a generic solution.
Edit: so to be clear, I'm looking for something like this:
#[1](true) ;; the anonymous function takes one parameter
回答1:
No. Just use fn. That's what it's for.
回答2:
If it's a constant return you are after you can use constantly:
(map (constantly 10) '(1 2 3 4 5))
; ==> (10 10 10 10 10)
If it's not you can use fn with a rest argument:
(fn [& _]
some-side-effect-expression)
; ==> non functional value
In fact this can be done on higher order functions where you don't need all the passes arguments:
(some-fun (fn [a b & _] (+ a b)) '(1 2 3 4 5))
; ==> (3 5 7 9)
回答3:
If you insist on the #( ... ) syntax, you can write
#(do % true)
... incurring no run-time penalty, whatever you want to return.
But (constantly true), as @Sylvester suggests, is idiomatic, and accepts any argument list:
((constantly true) 1 2 "Buckle my shoe");true
来源：https://stackoverflow.com/questions/41909132/declare-dummy-unused-parameters-for-clojure-anonymous-function