Clojure Data Structure Functions

Question

I'm new to Clojure and trying to learn the basics. One thing that tripped me up is understanding the correlation between the data structures and the functions they use.

For instance, if I create a new Vector:

(def my-vec [1 2 3])

Then when I try to call my-vec:

(my-vec)

I get:

ArityException Wrong number of args (0) passed to: PersistentVector  clojure.lang.AFn.throwArity (AFn.java:437)

I know that I can pass an argument and it appears to be calling get but how do I know? What args does PersistentVector take and where do I find documentation about it?

I tried:

(doc PersistentVector)

But that returns nil.

ChrisR · Accepted Answer · 2012-08-10 06:29:04Z

4

Documentation can be found under IPersistentVector here: http://clojure.org/data_structures

In particular: Vectors implement IFn, for invoke() of one argument, which they presume is an index and look up in themselves as if by nth, i.e. vectors are functions of their indices.

answered Aug 10, 2012 at 6:29

ChrisR

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2 Comments

James Ward Over a year ago

That helps! So when I do (my-vec) or (my-vec 1) it calls invoke() which for IPersistentVector calls nth?

James Ward Over a year ago

Found the answer in the code: github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/…

Arthur Ulfeldt · Accepted Answer · 2012-08-10 06:44:43Z

1

If you pass a number to a Clojure vector the vector will use that number as an index into it's self and return the value at that index:

user> (def my-vec [1 2 3 4 5])
#'user/my-vec
user> (my-vec 2)
3

this allows you to write expressions like this which grab several keys out of a vec

user> (map my-vec [1 3 4])
(2 4 5)

edited Aug 10, 2012 at 6:44

answered Aug 10, 2012 at 6:27

Arthur Ulfeldt

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3 Comments

Ankur Over a year ago

Hmmm I guess someone got confused with your statement 'look that number up' and that caused a down vote.. Please make it clear that the number is an index and not a value in vector

Arthur Ulfeldt Over a year ago

fixed :) though i suspect someone just down-voted all the answers in this question

James Ward Over a year ago

Sorry Arthur. I down voted this because it doesn't answer my actual question. My question wasn't about why I was getting an error. It was about how do I find what function is getting called. invoke() is the magic piece that I didn't know about.

rmm · Accepted Answer · 2019-10-28 23:52:21Z

-1

You can think of a vector as mapping indexes 0, 1, 2, ..., N to values, one at each index. Abstractly, it's a special case of a map where the keys are integers starting from 0. That notion helps to see the consistency in Clojure between maps and vectors when used as functions:

(<ILookup-able-collection> <key-for-lookup>)

JavaScript does something similar, allowing you to use [] syntax for lookup on arrays or objects.

answered Oct 28, 2019 at 23:52

rmm

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Comments

kelsar · Accepted Answer · 2012-08-10 06:17:26Z

-2

my-vec isn't a function, so you should call: my-vec not (my-vec)

Try: (nth my-vec i) for get i-th element of this vector.

link: nth

answered Aug 10, 2012 at 6:17

kelsar

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