On May 11, 1:39 pm, Alexandre Patry <patry...@iro.umontreal.ca> wrote:
> I am trying to call a java method using apply, like :
>
> (apply .println [System/out "hello" "world"])
>
> But I get an error: "Unable to resolve symbol: .println in this context"
>
> Am I missing something?
The semantics (meaning of the syntax/code) when using Java interop is
a little murky. Java interop is neither Java syntax nor Lisp code. It
is Java code written in Lisp syntax. In general, semantics of Java has
nothing to do with semantics of pure Clojure. They are worlds apart.
Apply method won't work in your example because it is a Clojure/Lisp
construct. It has meaning only in the Lisp world. It doesn't have an
equivalent in the Java world. Java interop is merely a way to write
Java code. When you invoke Java code in Clojure, it may help to
remember that you have stepped outside of pure Lisp/Clojure. You are
invoking Java code whenever you use a dot to prefix or suffix a word.
Except, of course, when you use memfn or anonymous function wrapping
over the java member function. When you do this, you enter a gray
zone, a haze really, in terms of simple semantics. This mechanism is a
bridge between the high-level language and the low-level language on
which the high-level language is built. The bridge is provided so that
the vast amount of libraries available for the JVM can be utilized
inside Clojure. This frees Clojure's implementor(s) to focus on higher
level constructs such as concurrency mechanisms, modern data
structures and so on.
Think of it this way - you know how sometimes, if people want to speed
things up in Java, they write native functions in C/C++? What if the
creators of Java had provided a means to call C/C++ with special
syntax from inside Java, instead of having to write and compile C++
separately in a different file? Not a completely correct analogy, but
I hope it is useful to some extent.
(.println System/out "1 2 3") is just another way of writing
(. System/out (println "1 2 3"))
which is just a convenient way of writing in Clojure, the following
line in Java.
System.out.println("1 2 3");
Just because you can invoke Java from Clojure does not mean you can
mix and match both languages freely inside the same form/expression
Are you familiar with Java and Lisp/Scheme? If not, when you find the
time, you could spend just enough time in each of the languages to
write a moderately sized application that invokes date/time functions,
vectors, arrays and file I/O. And then write the same application in
Clojure.
- Aravindh
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