Wednesday, March 10, 2010

Java access syntax from Jarc - less dots, more filling

Have you ever wondered if the ease of calling C libraries could be responsible for a lot of Python's popularity? C function calls look just like native Python calls.

import george;

You can't tell from the Python code whether the module, george, is written in C or Python. It doesn't matter to the calling program. That's a simple foreign function interface.

Jarc brings this same simple foreign function interface to Arc. Unlike Clojure and JScheme, the syntax for calling a Java method is the same as for calling any Lisp function.

Jarc> (getTime (new java.util.Date))
Jarc> (getTime "foo")
Error: Symbol 'getTime' has no value

Even though there is no function getTime defined, that function can still be called on a Date instance.

Jarc uses dispatch on first arg to figure out how to evaluate the method call. This was suggested by Paul Graham in Arc at 3 Weeks. Although Arc doesn't currently have dispatch on first arg it is ideal for Jarc to access Java methods.

If you've defined classes in Python (or Perl), this may seem intuitive.

class HelloClass:
    def f(self):
        return 'hello world'
That self there is the first argument to the function. Even though you call the function as x.f() what's happening under the covers is that x is passed as the first argument. This same thing is happening under the covers in Perl and C++.


1. You can treat Arc calls and Java calls exactly the same

Polymorphism, anyone? Here's the Jarc macro with-open, which is just like let except that it also calls close on variable. It is slightly more complicated then that because is always calls close even if there is an error. And it ignores any errors that might happen when calling close.

(mac with-open (var init . exprs)
   `(let ,var ,init
      (protect (fn () ,@exprs)
        (fn () (errsafe (close ,var))))))
This is quite handy and ensures that your "stream" gets closed, both when it is an arc type:
(with-open f (outfile "what.ever")
Where Jarc calls the Arc function close, and when the "stream" is a Java object:
(with-open db (java.sql.DriverManager.getConnection ...)
Where Jarc calls the close method on the java.sql.Connection instance.

2. Java objects work with map

No helper function (like memfn in Clojure) is needed to use Java instance methods with map.

(map 'getTime '(list (new java.util.Date)))
Both map and apply accept a symbol (in addition to a function, of course) and interpret that as a Java method call.

3. One less character

And of course, since succinctness is power, saving one whole character is an advantage as well. Clojure requires you to type an additional period.

(.getTime (new java.util.Date))
Astute readers will know that succinctness is defined by the number of nodes in the parse tree. And the Clojure example above still has the same number of nodes as the Jarc version. But the number of nodes is also a proxy for how hard it is to read the code. Our brains have to process the code too. And I think parsing .getTime requires parsing the dot separately. And it's not useful information. Just like in Python, I don't want to be distracted with extra syntax to indicate that this is a Java method call. It's just a function call and should be just as simple.


  1. Thanks a lot for the prompt and accurate response! ^^; Within hours of the question, I'd composed most of a fairly substantial response, but since then I've been too busy and/or tired to finish it up and post it. Thanks for taking the initiative to give the short version. :)
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  3. Hello. Just tried one of the examples above and got the following error. Just now downloaded Jarc21.

    Jarc> (map 'getTime '(list (new java.util.Date)))
    Can't find method: jarc.Symbol.list()
    (stdin):1: Error: Can't apply function: getTime

    0: (map (quote getTime) (quote (list (new java.util.Date))))
    DEBUG 0:

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