Search code examples
clojurefunctional-programming

Clojure: clojure.lang.ArityException when returning map from reader macro function

I can return a map from function with following syntax

(defn retmap [bar] { :foo bar })

How do I achieve the same with reader macro syntax? I tried following

(def retmap #({:foo %}))

But calling this function (retmap) gives the error

clojure.lang.ArityException: Wrong number of args (0) passed to: PersistentArrayMap
Solution

  • You can use hash-map:

    (def retmap #(hash-map :foo %))

    You can see why your example throws an exception by expanding the macro:

    (macroexpand `#({:foo %}))
=> (fn* [x] ({:foo x}))

    so the constructed map is immediately invoked as a function with no arguments. Maps are functions from keys to values so require an argument to be provided.