Here is a way that should work.
(let [missing (gensym)]
(defn get-with-exception [map key]
(let [res (get map key missing)]
(if (= res missing)
(throw (new Exception "my-exception"))
res))))
Gensyms are unique so you also don't have the problem of 'what happens
if I put an exception in the map' in this case.
You don't want to be executing the gensym more than once, or the
exception more than once.
Symbol comparison should also be reasonably fast, which is good.
On Mar 20, 5:50 am, Andreas Kostler <andreas.koestler.le...@gmail.com>
wrote:
> Hi all,
> I would like to throw an exception when I'm trying to retrieve a value in a
> map for a key that doesn't exist.
> The obvious first shot was:
> (get {:foo "bar"} :foo (throw (new Exception "Oh no!")))
> However, this doesn't work because the exception always throws since get
> apparently eagerly evaluates it's arguments.
> One way to work around this could be:
>
> (defn get-with-exception [map key]
> (let [res (get map key (new Exception "my-exception"))]
> (if (= (class res) java.lang.Exception)
> (throw res)
> res)))
>
> I was wondering, is there another more concise/idiomatic way?
>
> Cheers
> Andreas
>
> --
> "Test-driven Dentistry (TDD!) - Not everything should be test driven"
> - Michael Fogus
> --
> **********************************************************
> Andreas Koestler, Software Engineer
> Leica Geosystems Pty Ltd
> 270 Gladstone Road, Dutton Park QLD 4102
> Main: +61 7 3891 9772 Direct: +61 7 3117 8808
> Fax: +61 7 3891 9336
> Email: andreas.koest...@leica-geosystems.com
>
> ************www.leica-geosystems.com*************
>
> when it has to be right, Leica Geosystems
>
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