Question

conj is used in (conj coll item), for example (conj #{} “Stu”).

But, I found this example in ‘Programming Clojure’ book page16.

(alter visitors conj username)

I guess (conj visitors username) would be the correct usage. What’s the secret?

Answer 1

The “secret” is that alter is a special function that is called in a transaction on a ref, and it has the effect of inserting the current value of the ref as the first parameter to whichever function is supplied as its second argument.

In fact the normal usage for conj with a ref would be:

(conj @visitors username)

So alter roughly translates as follows:

(alter vistors conj username)
=> (ref-set visitors (conj @visitors username))