On Mon, Dec 15, 2008 at 9:43 PM, Daniel Eklund <doekl...@gmail.com> wrote:
>
> > Looks like an if then else version of the map lookup??
> > ie: (if (%1 %2) (%1 %2) %3)
> > Is this a special feature of maps in general, such that you can look
> > up a key but return something else if it doesn't exist?
> > I hadn't come across it yet, but it sounds useful :)
>
> This is exactly right (I just confirmed it for myself). I looked up
> the java code for Keyword.java and Symbol.java both of which implement
> IFn (those things that can be put in the function position). The
> single and double arity invoke() merely delegate to 'get'. And voila:
>
How did you know that it delegates to 'get'?
>
> user> (doc get)
> -------------------------
> clojure.core/get
> ([map key] [map key not-found])
> Returns the value mapped to key, not-found or nil if key not
> present.
>
> The last parameter is definitely for those situations where 'if not
> found, use this'
>
> I think I _finally_ understand the statement, "keywords and symbols
> are functions of maps".
>
> So, the original poster's observation about ('+ '1 '2) returning '2
> makes more sense when you consider that it turns into this:
>
> (get '1 '+ '2)
>
> or "get from the hashmap ( '1 ) the value stored at key ( '+ ) and if
> nil is returned, return ( '2 ) instead"
>
> (get '1 '+) returns nil as '1 is not even a map.
>
>
> >
>
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