Exception using map and anonymous functions in Clojure

Question

I was messing around with Clojure maps and I discovered this situation that I can't understand.

Let's say I got a map like this:

(def map-test {:name "head" :size 3})

I want to change the value of this map, in Clojure the usual way is to generate a new one with the modified data.

So I have this function:

(defn map-change
  [part]
  {:name (str (:name part) "-" 1) :size (:size part)})

As expected the invocation (map-change map-test) returns: {:name "head-1", :size 3}

So I wrote this function using map to clone the hash-map a given number of times, like this {:name "head-1" ...}{:name "head-2" ...}{:name "head-3" ...} etc:

(defn repeat-test
  [part times]
  (map #({:name (str (:name part) "-" %) :size (:size part)}) (range 1 (inc times))))

But I got an exception that I cannot understand when I call (repeat-test map-test 5):

Wrong number of args (0) passed to: PersistentArrayMap

The debugger throws this exception when is assigning the value to :size right after has evaluated (:size part)=>3

Here's the last part of the stacktrace:

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

                  AFn.java:  429  clojure.lang.AFn/throwArity
                  AFn.java:   28  clojure.lang.AFn/invoke
                      REPL:   80  clj-lab-00.hobbits/repeat-test/fn
                  core.clj: 2644  clojure.core/map/fn
              LazySeq.java:   40  clojure.lang.LazySeq/sval
              LazySeq.java:   49  clojure.lang.LazySeq/seq
                   RT.java:  521  clojure.lang.RT/seq
                  core.clj:  137  clojure.core/seq
            core_print.clj:   46  clojure.core/print-sequential
            core_print.clj:  153  clojure.core/fn
            core_print.clj:  153  clojure.core/fn
              MultiFn.java:  233  clojure.lang.MultiFn/invoke
                  core.clj: 3572  clojure.core/pr-on
                  core.clj: 3575  clojure.core/pr
                  core.clj: 3575  clojure.core/pr
                  AFn.java:  154  clojure.lang.AFn/applyToHelper
....

But if I use a non anonymous function that does the same operation as the anonymous one:

(defn map-change
  [part i]
  {:name (str (:name part) "-" i) :size (:size part)})

(defn repeat-test
  [part times]
  (map #(map-change part %1) (range 1 (inc times))))

Now calling (repeat-test map-test 5) works. Why ? What am I missing ?

Michiel Borkent · Accepted Answer · 2016-10-08 21:44:32Z

6

The error is similar to this simplified example:

(map #({:a %}) [1 2 3])

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

You can expand #({:a %}) to see what code is actually being compiled and executed:

(macroexpand '#({:a %}))
;;=> (fn* [p1__21110#] ({:a p1__21110#}))

In other words, #({:a %}) expands to something like (fn [x] ({:a x})). The problem in the body of this function is that the map is called as a function, with no arguments.

Maps behave like functions: they are functions of their keys. But they expect at least one argument and at most two:

({:a 1} :a) ;;=> :a
({:a 1} :b 2) ;;=> 2

You didn't intend to call the map as a function at all. You just wanted to have the map. The anonymous function literal always expands into a function call and cannot yield a direct value. You can solve this several ways:

#(-> {:a %})
#(identity {:a %})
#(hash-map :a %)
#(do {:a %})
(fn [x] {:a x})

I prefer the latter, although I find the first one pretty funny. It's like saying: I want to return the thing I'm pointing at.

edited Oct 8, 2016 at 21:44

answered Oct 8, 2016 at 21:12

Michiel Borkent

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3 Comments

OlegTheCat Over a year ago

I'd add #(do {:a %}) as an option.

Marcs Over a year ago

So the rule of thumbs for a beginner is to use (fn [x] ...) and avoid the #() form at least until I'll understand macros better.

Michiel Borkent Over a year ago

@OlegTheCat That's also a possibility. I added it. There are many more options: #(and {:a %}), #(or {:a %}) :-). @Marcs I think that is a good rule of thumb. One other point to note: anonymous function literals can't be nested.

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