On 29 October 2014 11:01, Roelof Wobben <[email protected]> wrote:
>
> For a exercise I have to add something to the end of a existing map.
>
> So I thought this would work :
>
> (defn add-author [book new-author]
> (assoc book (conj :authors new-author)))
>
Take a look at that conj expression on its own:
(conj :authors new-author)
You're trying to conjoin "new-author" onto the keyword :authors, but
keywords aren't collections. That's what the error means. It's saying that
it expected a collection, but you've supplied a keyword instead.
What you want is:
(conj (get book :authors) new-author)
Which can also be written:
(conj (book :authors) new-author)
(conj (:authors book) new-author)
These two expressions are shortcuts for the above "get" function. They work
because maps and keywords can act as functions.
- James
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