Symbols and keywords act as functions which look themselves up in their
argument. Essentially:
('sym map) => (get map 'sym)
('sym map not-found) => (get map 'sym not-found)
(:key map) => (get map :key)
(:key map not-found) => (get map :key not-found)
It's often pretty useful, in my experience. It looks cleaner, and it means
that you can just use :keyword instead of #(get % :keyword) in cases where
you provide a function as an argument (for example: (map :type objects)).
On 16 September 2015 at 14:20, Michael O'Keefe <michael.p.oke...@gmail.com>
wrote:
> Hello All:
>
> Noticed the following works in Clojure 1.6 and 1.7 (didn't test prior to
> that):
>
> user=> ('+ 'a 'b)
> b
>
> Also "works" for any set of symbols:
>
> user=> ('abra 'ka 'dabra)
> dabra
>
> My expectation was that using symbols or keywords in the function/operator
> position would throw an exception similar to this:
>
> user=> (3 4)
>
>
> ClassCastException java.lang.Long cannot be cast to clojure.lang.IFn user
> /eval7668 (form-init2947310628177413729.clj:1)
>
>
> I noticed Keywords can also be used in the operator position:
>
> user=> (:abra :ka :dabra)
> :dabra
>
> And mixing and matching is OK:
>
> user=> (:a 'b 3)
> 3
>
> It seems to act something like an implicit `do` in that it returns the
> last value but, in fact, not quite as you seem to need to have more than
> two items:
>
> user=> ('do)
>
> ArityException Wrong number of args (0) passed to: Symbol
> clojure.lang.AFn.throwArity (AFn.java:429)
>
>
> user=> ('do (+ 1 2 3))
> nil
>
>
> user=> ('do 'this (+ 1 2 3))
> 6
>
> But more than 3 is not accepted:
>
> user=> ('do 'this 'please 3)
>
>
> ArityException Wrong number of args (3) passed to: Symbol
> clojure.lang.AFn.throwArity (AFn.java:429)
>
> user=> ('do 'this 'please 'now 'ok?)
>
>
> ArityException Wrong number of args (4) passed to: Symbol
> clojure.lang.AFn.throwArity (AFn.java:429)
>
>
> A quick search didn't reveal any bug reports or previous discussions on
> this but I may have missed something (feel free to point me in the right
> direction).
>
> Is it just a "quirk" or is it useful in some way? Intended behavior or bug?
>
> Cheers,
>
> Michael O'Keefe
>
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