Clojure custom function for threading macro

I have a map and I want to write a custom function for updating it.

(-> {:a 1 :b 2}
    (fn [x] (update x :a inc)))

This of course is a simple example and could be easily done without the function wrapped around the update, but it shows what I want to do. But this gives me the following error.

Syntax error macroexpanding clojure.core/fn at (core.clj:108:1).
{:a 1, :b 2} - failed: vector? at: [:fn-tail :arity-1 :params] spec: :clojure.core.specs.alpha/param-list
{:a 1, :b 2} - failed: (or (nil? %) (sequential? %)) at: [:fn-tail :arity-n] spec: :clojure.core.specs.alpha/params+body

I don't get why this is not working, since the threading macro should but my map as first parameter in the function, right?

Solution

You can always use macroexpand to see what happened. In your case, macroexpand will return you:

(fn {:a 1, :b 2} [x] (update x :a inc))

obviously this is not a valid function. But if you tweak it this way:

(-> {:a 1 :b 2}
    (#(update % :a inc)))

the expanded form will then become valid:

(#(update % :a inc) {:a 1, :b 2})