use macros to generate java calls

[Jamison Hope]

On Dec 5, 2011, at 3:47 AM, Per Bothner wrote:

> On 12/05/2011 12:30 AM, Răzvan Rotaru wrote:
>> Hi,
>>
>> I'm trying to use macros to generate java calls, and have been
>> strugling with this all weekend, but with no success. So, I'd like to
>> ask for some help from more experienced schemers:
>>
>> The macro should work like this:
>>
>> (create-object obj-type 'id1 arg1 'id2 arg2 ...)
>>
>> =>
>>
>> (obj-type) ;instantiation
>> (*:setId1 arg1)
>> (*:setId2 arg2)
>> ...
>>
>>
>> Is this possible with kawa? Any hints how to achieve this? The  
>> biggest
>> challenge will probably be to transform 'id-foo in setIdFoo.
>
> Rather than write your own macro, I suggest using the builtin  
> facilities:
> http://www.gnu.org/software/kawa/Allocating-objects.html
>
> That makes use of keywords, changing your example to:
>
> (obj-type id1: arg1 id2: arg2 ...)

Răzvan, out of curiosity, what problems were you facing? Using the  
built-in
syntax is better in this case, but it might still be a good learning  
experience
for writing macros. As for the symbol transformations, instead of  
trying to
turn 'id-foo into "setIdFoo", I'd probably turn it into "*:set-id-foo"  
and
rely on Kawa's magic to convert that to CamelCase. (Note that the  
built-in
object creation syntax is more robust, since it'll turn "id1:" into  
either
setId1() or addId1() depending upon which method actually exists.)

Of course, there are times when the built-in facility isn't  
applicable, like
if you can't invoke a constructor (e.g. have to retrieve a singleton  
or call
a factory method) or if the methods you want to invoke on the object  
aren't
add/set or take more (or less) than one argument. For those cases, I  
wrote a
Kawa version of Clojure's doto macro
http://clojure.org/java_interop
which works pretty nicely:

(doto (java.util.HashMap) (*:put "a" 1) (*:put "b" 2))
=> {b=2, a=1}

(doto java.lang.System:out (*:println "Hello") (*:println "World"))
Hello
World
=> java.io.PrintStream@4e76fba0

The guts of it are fairly straightforward, so it makes for a decent  
macro
programming exercise. (And it doesn't require any symbol manipulation  
a la
id-foo -> *:set-id-foo, so it's probably an easier starting point than  
your
create-object.)

-Jamie

--
Jamison Hope
The PTR Group
www.theptrgroup.com