Hi Roman!
On Tue, Sep 22, 2009 at 2:58 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)
>
because -> put its firs t argument in second position into its second
argument:
(-> foo (fn [x] x)) becomes (fn foo [x] x)
user=> (macroexpand-1 '(-> foo (fn [x] x)))
(fn foo [x] x)
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)
>
Ditto. At read time, #(...) expand to seomething like (fn [x] (...)).
One usual albeit somewhat ugly workaround is to put another pair of parens
around the fn
user=> (macroexpand-1 '(-> foo ((fn [x] x))))
((fn [x] x) foo)
HTH,
Christophe
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