clojure

Anonymous function error: class java.lang.Long cannot be cast to class clojure.lang.IFn

user=> ((fn [x] x) 5)
5
user=> (#(%) 5)
Execution error (ClassCastException) at user/eval1576$fn (REPL:1).
class java.lang.Long cannot be cast to class clojure.lang.IFn (java.lang.Long is in module java.base of loader 'bootstrap'; clojure.lang.IFn is in unnamed module of loader 'bootstrap')

I'd expected the two anonymous funtions to be equivalent. I kind of get the error in the second case: the evaluation of the anonymous function yields (5), which produces the exact same error.

However, why am I not getting this error in the first case? What's the difference?

Solution

  • #(...), when there's only % inside and not other things that signify more than one argument, is equivalent to (fn [%] (...)). Not to (fn [%] ...), which you expected.

    So #(%) is the same as (fn [%] (%)) - it's a function that calls its argument as a function without arguments.