# Why here a arity errror and how to solve it
**URL:** https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010
**Category:** Beginners
**Created:** [January 9, 2021, 12:08pm UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010 "2021-01-09T12:08:25Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@RoelofWobben](https://clojureverse.org/u/RoelofWobben)
#### Post date: [January 9, 2021, 12:08pm UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010/1 "2021-01-09T12:08:25Z")
I try to implement my own update-in as challenge from the brave book
so far I have this :
```auto
(def m {:1 {:value 0, :active false}, :2 {:value 0, :active false}})
(defn my-update-in [m keyword number function]
(function (get-in m [(number keyword) keyword]) ))
(update-in m [:1] assoc :value 1 :active true)
(defn add-ten [number]
(+ number 10))
(my-update-in p :age add-ten)
```
but now I see a arity error on the last one.
So how I can I make the code work in both cases
---
### Author:  [@Phill](https://clojureverse.org/u/Phill)
#### Post date: [January 9, 2021, 12:38pm UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010/2 "2021-01-09T12:38:15Z")
How many parameters does my-up-date-in require? How many parameters did the invocation provide? That’s the arity error.
Also, keep in mind an interesting subtlety of the real update-in, which is, that its return value is **not** the return value of the function that the caller provides!
---
### Author:  [@RoelofWobben](https://clojureverse.org/u/RoelofWobben)
#### Post date: [January 9, 2021, 12:41pm UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010/3 "2021-01-09T12:41:46Z")
I thought that already
so I cannot make one function which takes both cases
---
### Author:  [@mvarela](https://clojureverse.org/u/mvarela)
#### Post date: [January 9, 2021, 4:14pm UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010/4 "2021-01-09T16:14:08Z")
Have a look at the variadic functions section here: [https://clojure.org/guides/learn/functions](https://clojure.org/guides/learn/functions)
Also, as @Phill pointed out, you should return the updated map, not the updated value itself
---
### Author:  [@RoelofWobben](https://clojureverse.org/u/RoelofWobben)
#### Post date: [January 9, 2021, 8:22pm UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010/5 "2021-01-09T20:22:40Z")
Get some help and I think I solved it
```auto
(defn my-update-in [m ks f]
(let [up (fn up [m ks f]
(let [k (first ks)
r (rest ks)]
(if (empty? r)
(assoc m k (f (get m k )))
(assoc m k (up (get m k) r f )))))]
(up m ks f)))
```
satisfied ??
---
### Author:  [@mvarela](https://clojureverse.org/u/mvarela)
#### Post date: [January 10, 2021, 8:03am UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010/6 "2021-01-10T08:03:02Z")
It’s almost there:
```
(my-update-in {:a {:b 1}} [:a :b] inc)
;; => {:a {:b 2}}
(my-update-in {:a {:b 1}} [:a :b] + 3)
;; => throws arity exception
(update-in {:a {:b 1}} [:a :b] + 3)
;; => {:a {:b 4}}
```
---
### Author:  [@RoelofWobben](https://clojureverse.org/u/RoelofWobben)
#### Post date: [January 10, 2021, 10:15am UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010/7 "2021-01-10T10:15:49Z")
This better
```auto
(let [up (fn up [m ks f args]
(let [k (first ks)
r (rest ks)]
(if (empty? r)
(assoc m k (apply f (get m k) args))
(assoc m k (up (get m k) r f args)))))]
(up m ks f args)))
```
---
### Author:  [@mvarela](https://clojureverse.org/u/mvarela)
#### Post date: [January 11, 2021, 7:22am UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010/8 "2021-01-11T07:22:56Z")
Have you had a look at the varargs section in the link I sent you? `update-in` should take the map, path to update, a function, and any number of arguments to that function.
What you have there is almost verbatim the `clojure.core/update-in` body, but missing the varargs part.
---
### Author:  [@RoelofWobben](https://clojureverse.org/u/RoelofWobben)
#### Post date: [January 11, 2021, 7:50am UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010/9 "2021-01-11T07:50:57Z")
yes, I have
```auto
(defn infix2 [[first-number operator second-number & r]]
(if (empty? r)
(list operator
(if (list? first-number)
(infix2 first-number)
first-number)
(if (list? second-number)
(infix2 second-number)
second-number))
(if (< (get priorities operator) (get priorities (first r)))
(list operator first-number (infix2 (conj r second-number)))
(infix2 (conj r (list operator first-number second-number))))))
```
this one works also on nested ones like `( ((4 - 2) +2) * 3)`
---
### Author:  [@system](https://clojureverse.org/u/system)
#### Post date: [July 12, 2021, 7:51pm UTC](https://clojureverse.org/t/why-here-a-arity-errror-and-how-to-solve-it/7010/10 "2021-07-12T19:51:00Z")
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