On Sep 22, 3:58 pm, Roman Roelofsen <roman.roelof...@googlemail.com>
wrote:
> Hi there!
>
> Lets assume I have this map:
>
> user=> (def person {:name "Father" :childs [{:name "Son" :age 10}]})
>
> Testing:
>
> user=> (-> person :childs first)
> {:name "Son", :age 10}
>
> Now lets filter the child map:
>
> user=> (def only-name (fn [m] (select-keys m [:name])))
> user=> (-> person :childs first only-name)
> {:name "Son"}
>
> So why does this not work?:
>
> user=> (-> person :childs first (fn [m] (select-keys m [:name])))
> java.lang.RuntimeException: java.lang.IllegalArgumentException: Don't
> know how to create ISeq from: Symbol (NO_SOURCE_FILE:59)
>
> Or this?:
>
> user=> (-> person :childs first #(select-keys % [:name]))
> java.lang.ClassCastException: clojure.lang.Symbol cannot be cast to
> clojure.lang.IPersistentVector (NO_SOURCE_FILE:60)
You assume more of -> than you should :)
-> does not take functions as parameters. It only sees a bunch of
symbols, as it is a macro, which means it does a code transformation.
It puts the first form (the symbol person) in the second position of
the following form (the keyword :childs), which must be a sequence, or
if it is not, is wrapped in a list first. This repeats recursively
until there are no more forms left.
Now, the anonymous function example fails because (-> person :childs
first (fn [] whatever)) transforms into (fn (first (:childs person)) x
[] y). which is not what you intended, but nonetheless a correct
result.
If you look at the examples that work and macroexpand them, you would
see that (-> person :childs first) transforms first into (-> (:childs
person) first) and from there to (first (:childs person)) which
happens to be a valid Clojure expression
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