Thanks all,
It clear to me that I have to read the explanation or the right wiki page
more carefully.
I tried all of them and they all give the same answer :
{:title "zbook", :authors [{:name "James"} {:name "Joe"}]}
Roelof
Op woensdag 29 oktober 2014 13:34:32 UTC+1 schreef François Rey:
> (oops my earlier message isn't quite right, here's the correct one)
>
> You also need to clarify your intention: are you adding an author to a
> list of authors, or setting the single author?
> I suppose the first, but then you need to be clear on what the list of
> authors is.
> In idiomatic clojure you'll want it to be a vector of authors, so if an
> author is a map like {:name "Gerald J. Sussman"}, then :authors must be
> associated to a vector of maps.
> Also bear in mind that assoc is geared towards associative data structures
> such as maps, while conj is geared towards sequential data structures such
> as vectors.
> That being said, you can still use assoc on vectors (indices are the keys)
> and use conj on maps (with a vector in the form of [key value]), but that's
> polymorphism convenience, not idiomatic.
> In the end add to a new author, you need to do:
> (assoc book :authors (conj (get book :authors) new-author))
> Which can also be made more idiomatic as already mentioned:
> (update-in book [:authors] assoc new-author)
> The twisted way of doing it would be:
> (conj book [:authors (conj (:authors book) new-author)])
>
> Try this in a REPL:
>
> (def book {:title "zbook" :authors [{:name "James"}]})
> (def new-author {:name "Joe"})
> (assoc book :authors (conj (get book :authors) new-author))
> (update-in book [:authors] conj new-author)
> (conj book [:authors (conj (:authors book) new-author)])
>
>
>
>
> On 29/10/14 12:20, Roelof Wobben wrote:
>
> Thanks James,
>
> But how do I use assoc with it
>
> I tried this :
>
> (defn add-author [book new-author]
> (assoc book (conj (book :authors) new-author)))
>
> but then I see this message :
>
>
> ArityException Wrong number of args (2) passed to: core$assoc
> clojure.lang.AFn.throwArity (AFn.java:437)
>
> Roelof
>
>
> Op woensdag 29 oktober 2014 12:08:35 UTC+1 schreef James Reeves:
>
>> 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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