How can I pass in the list of methods to gen-class?

Question

When using gen-class this compiles fine:

(ns clj.sandbox)

(defn -hello
  [this]
  "Hello World")

(gen-class
  :name com.sandbox.GeneratedClass
  :methods [[hello [] String]])

But if you do this:

(ns clj.sandbox)

(def my-methods (atom [[hello [] String]]))

(defn -hello
  [this]
  "Hello World")

(gen-class
  :name com.sandbox.GeneratedClass
  :methods @my-methods)

You get this error:

CompilerException java.lang.RuntimeException: Unable to resolve symbol: hello in this context, compiling:(clj\sandbox.clj:3:17)

Is there any way to get around this error? I'd like to be able to pass in the :methods value instead of defining it inline.

In case it matters, this is the pom.xml I'm using to generate this:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">    
    <artifactId>sandbox</artifactId>
    <groupId>com.sandbox</groupId>
    <version>1.0-SNAPSHOT</version>    
    <packaging>clojure</packaging>
    <modelVersion>4.0.0</modelVersion>    
    <build>        
        <plugins>
            <plugin>
                <groupId>com.theoryinpractise</groupId>
                <artifactId>clojure-maven-plugin</artifactId>
                <version>1.3.13</version>
                <extensions>true</extensions>
                <configuration>
                  <sourceDirectories>
                    <sourceDirectory>src</sourceDirectory>
                  </sourceDirectories>
                </configuration>
            </plugin>
        </plugins>        
    </build>
    <dependencies>        
        <dependency>
            <groupId>org.clojure</groupId>
            <artifactId>clojure</artifactId>
            <version>1.5.1</version>
        </dependency>
    </dependencies>
</project>

I took off the "-" behind the hello function, so I had (defn hello [this] ...) and there was no error. — dizzystar, Apr 24 '13 at 18:35
@dizzystar yes, that will run, but it won't be able to generate the class because it won't be able to find the implementation. — Daniel Kaplan, Apr 24 '13 at 19:37
Perhaps look into multimethods. http://clojure.org/multimethods and the cheatsheet is always a life-saver: http://clojure.org/cheatsheet. Hope that helps, but I'm not entirely sure what you are trying to accomplish. — dizzystar, Apr 25 '13 at 03:48
gen-class is a macro, and expects things like the method list to be available at compile time. You could perhaps compute your method list and expand it into a gen-class form via your own macro, but I'm not sure yet why you're wanting you do this at all. — Chouser, Apr 25 '13 at 05:18
@Chouser: I was hoping to make a library that takes clojure code that looks (perhaps exactly) like compojure routes and generates it into Java classes that implement Jersey code. I wanted each `GET` you write to generate a method in a Jersey class. Here's a page to show you what Jersey source looks like (it's simple): https://jersey.java.net/nonav/documentation/latest/user-guide.html#d4e47 @amalloy already told me I should do this in Java because it's too much trouble in clojure, but that was after I wrote this Q. I'd like to know more/if you agree. I already figured out the annotation part — Daniel Kaplan, Apr 25 '13 at 06:59
Don't let cranky old men like @amalloy ruin your dreams. :-) If you succeed, the results may not be anything you'd actually want to use, but the journey getting there will doubtless be instructive. You will need to write a macro, doubtless a rather complex one. I'd recommend writing out the kind of complex gen-class form you want to produce, and the equivalent macro call that should produce it. Finally, write plain functions that convert from one to the other and a tiny macro to call those functions. Or find something more practical to do with your time. :-) Either way, good luck! — Chouser, Apr 26 '13 at 04:27

score 2 · Accepted Answer · answered Apr 25 '13 at 05:31

2

Because gen-class is a macro and passing @my-methods to it will cause the macro to get (deref my-method) as value of method argument which is not what is expected. You need to create a wrapper macro and then call it as shown below:

(defn -hello []
  "Hello world")

(def my-methods (atom '[[hello [] String]]))

(defmacro my-class []
  `(gen-class
    :name com.sandbox.GeneratedClass
    :methods ~(deref my-methods)))

(my-class)

Also note that the atom value is quoted otherwise you will get hello not found exception because it tries to resolve hello var which is not there.

answered Apr 25 '13 at 05:31

Ankur

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OK thanks a lot for the reply. I'll try that out very soon. Out of curiosity, how could I have debugged that myself? I tried running `macroexpand-1` but it just returned `nil`. – Daniel Kaplan Apr 25 '13 at 07:02
`gen-class` macro doesn't emit any code, it just execute some code that create the class file and return nil – Ankur Apr 25 '13 at 07:10

How can I pass in the list of methods to gen-class?

1 Answers1