Hey Ru,
I'd also add that `new` is a special form, not a function. It is evaluated
at compile-time, which is why the macro doesn't work. You can also try
macro-expansion to see what's going on:
(defn itest [x] (macroexpand-1 '(instance x)))
=> #'user/itest
(itest java.lang.String)
=> (new x)
So, after macro-expansion, this would be like typing the following code:
(defn itest [x] (new x))
Syntax error (IllegalArgumentException) compiling new at [...]
Unable to resolve classname: x
This doesn't work as a straight up function, so it won't work as a macro.
Macros are "shortcuts". They won't let you do anything you couldn't do
with "regular" code.
Finally, as Chris suggested above, reflection is probably what you want.
Here's a sample to get you started:
(defn itest [x & args]
(let [cnstr (.getConstructor x (into-array java.lang.Class [String]))]
(.newInstance cnstr (into-array java.lang.Object args))))
=> #'user/itest
(itest java.lang.String "Hello")
=> "Hello"
This is only a start though, because as written (line 2 in particular), it
only works for constructors that take exactly one String argument. To do
this for real, you would need to build the [String] array dynamically by
introspecting the args list.
Marc
On Sat, Oct 19, 2019 at 3:02 PM <r...@chartbeat.com> wrote:
> Macros also allow you to create a dsl to make your code easier to use or
> understand by obfuscating away bits. For example a macro that times a
> function call by proxying the call or wrapping a call to defn.
>
> That said, I consider many uses of macros that I’ve seen to be
> antipatterns that lead to hard to understand code. They should almost
> always be considered a last resort.
>
>
> On Oct 19, 2019, at 12:12 PM, Matching Socks <phill.w...@gmail.com> wrote:
>
> Macros manipulate program symbols. Macros fill the role that is filled by
> Perl scripts in the Java world, when they pre-process stuff (a database
> schema, or a gui model, for example) into actual Java code before you
> compile it. If a task could not be solved by pre-processing the source
> code before the program starts to run, then it is not a task for a macro.
>
> On the bright side, using Java reflection is less tedious in Clojure than
> it is in Java.
>
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