On Tue, Jul 26, 2011 at 3:28 PM, Alan Malloy <a...@malloys.org> wrote:
> On Jul 25, 11:10 pm, Petr Gladkikh <petrg...@gmail.com> wrote:
>> I am trying to construct java object and assign it's fields from a map.
>> That is given Java object of class Something { long id; String name; }
>> and Clojure map {:id 12, :name "Impostor"}
>> I would like to set fields of java object to respective values from map.
>>
>> Now, this works
>> (let [o (Something.)]
>> (set! (. o :id) 12)
>> (set! (. o :name) "Impostor")
>> o)
>>
>> But as soon as I use some value to get field expression compiler
>> starts complaining "Invalid assignment target".
>> that is
>> (let [o (Something.)
>> ff :id]
>> (set! (. o ff) 12)
>> o)
>> I do not understand why this problem occurs. Any variations that I
>> tried to made it work do not do the trick.
>> Including weird (or ridiculous) ones like (set! (. o (symbol (str ":"
>> (name ff))) 12)
>>
>> I suspect that this has something to do with compiler that needs field
>> names at compile time but Clojure could use reflection for this...
>>
>> Can anyone point to what is wrong here?
>>
>> By the way is there already some function that allows to set fields of
>> an object from a map?
>
> Clojure *could* use reflection to do this...unless your object had a
> field named ff! It has to decide at compile time how to look up a
> field, and at that time it doesn't know your object won't have a .ff
> field, so it figures, sure, I'll set the ff field.
>
> If you really want to do this (hint: you don't), you can manually deal
> with the reflection that the compiler would generate, as Shantanu
> outlines.
Could you elaborate on this? What would you use instead in this case?
My motivation is need to construct list of Java objects and I would
like to have some concise syntax to write them. So I decided to do
this with maps.
I wrote a function that acts as constructor. But long list of
(set! (. obj :aa) (:aa props))
(set! (. obj :bb) (:bb props))
(set! (. obj :cc) (:cc props))
(set! (. obj :dd) (:dd props))
looks not very lispy. Maybe I should use macros instead?
--
Petr Gladkikh
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