Hi Vincent,
`defprotocol` is a top-level form, not really meant to be mixed with
value-returning expressions like `fn`. Protocols are always global because
of how they compile into Java interfaces.
Here's one way to make it work, by defining a symbol instead of returning a
function:
(defmacro create-protocol [protocol symbol implementation]
(let [[protocol-name signature] protocol]
`(do
(defprotocol ~protocol-name ~signature)
(defn ~symbol [] (reify ~protocol-name ~implementation)))))
(create-protocol [P (method [this])]
constructor
(method [this] (println "method")))
(method (constructor))
-S
On Wednesday, June 5, 2013 9:16:05 AM UTC-4, Vincent wrote:
>
> I’m trying to write a macro that defines a protocol and a function that,
> when called, returns an implementation of that protocol.
>
> I’ve reduced the code to the following example:
> (defmacro create-protocol [protocol implementation]
> (let [[protocol-name signature] protocol]
> `(do
> (defprotocol ~protocol-name ~signature)
> (fn [] (reify ~protocol-name ~implementation)))))
>
> (let [f (create-protocol [P (method [this])]
> (method [this] (println "method")))]
> (method (f)))
>
>
> The original code is more complicated, where the function will read a
> value from a file and, depending on that value, return the appropriate
> implementation of the protocol.
>
> When I run Clojure 1.5.1 on that code I get the following exception:
> Exception in thread "main" java.lang.NullPointerException,
> compiling:(protocol.clj:7:9)
> at clojure.lang.Compiler.analyzeSeq(Compiler.java:6567)
> at clojure.lang.Compiler.analyze(Compiler.java:6361)
> at clojure.lang.Compiler.analyzeSeq(Compiler.java:6548)
> at clojure.lang.Compiler.analyze(Compiler.java:6361)
> at clojure.lang.Compiler.analyze(Compiler.java:6322)
> at clojure.lang.Compiler$BodyExpr$Parser.parse(Compiler.java:5708)
> at clojure.lang.Compiler$FnMethod.parse(Compiler.java:5139)
> at clojure.lang.Compiler$FnExpr.parse(Compiler.java:3751)
> at clojure.lang.Compiler.analyzeSeq(Compiler.java:6558)
> at clojure.lang.Compiler.analyze(Compiler.java:6361)
> at clojure.lang.Compiler.analyzeSeq(Compiler.java:6548)
> at clojure.lang.Compiler.analyze(Compiler.java:6361)
> at clojure.lang.Compiler.analyze(Compiler.java:6322)
> at clojure.lang.Compiler$BodyExpr$Parser.parse(Compiler.java:5708)
> at clojure.lang.Compiler.analyzeSeq(Compiler.java:6560)
> at clojure.lang.Compiler.analyze(Compiler.java:6361)
> at clojure.lang.Compiler.analyzeSeq(Compiler.java:6548)
> at clojure.lang.Compiler.analyze(Compiler.java:6361)
> at clojure.lang.Compiler.access$100(Compiler.java:37)
> at clojure.lang.Compiler$LetExpr$Parser.parse(Compiler.java:5973)
> at clojure.lang.Compiler.analyzeSeq(Compiler.java:6560)
> at clojure.lang.Compiler.analyze(Compiler.java:6361)
> at clojure.lang.Compiler.analyze(Compiler.java:6322)
> at clojure.lang.Compiler$BodyExpr$Parser.parse(Compiler.java:5708)
> at clojure.lang.Compiler$FnMethod.parse(Compiler.java:5139)
> at clojure.lang.Compiler$FnExpr.parse(Compiler.java:3751)
> at clojure.lang.Compiler.analyzeSeq(Compiler.java:6558)
> at clojure.lang.Compiler.analyze(Compiler.java:6361)
> at clojure.lang.Compiler.eval(Compiler.java:6616)
> at clojure.lang.Compiler.load(Compiler.java:7064)
> at clojure.lang.Compiler.loadFile(Compiler.java:7020)
> at clojure.main$load_script.invoke(main.clj:294)
> at clojure.main$script_opt.invoke(main.clj:356)
> at clojure.main$main.doInvoke(main.clj:440)
> at clojure.lang.RestFn.invoke(RestFn.java:408)
> at clojure.lang.Var.invoke(Var.java:415)
> at clojure.lang.AFn.applyToHelper(AFn.java:161)
> at clojure.lang.Var.applyTo(Var.java:532)
> at clojure.main.main(main.java:37)
> Caused by: java.lang.NullPointerException
> at clojure.lang.Compiler.resolveIn(Compiler.java:6840)
> at clojure.lang.Compiler.resolve(Compiler.java:6818)
> at clojure.lang.Compiler$NewInstanceExpr.build(Compiler.java:7427)
> at
> clojure.lang.Compiler$NewInstanceExpr$ReifyParser.parse(Compiler.java:7377)
> at clojure.lang.Compiler.analyzeSeq(Compiler.java:6560)
> ... 38 more
>
> From what I understood by tracing through the compiler it seems that the P
> var is created at macro expansion time but bound at execution time only.
> When expanding the ‘reify’ macro, P is still unbound, which yields a nil
> interface, which triggers the NPE.
>
> I could solve the problem by redefining the macro in the following way:
> (defmacro create-protocol [protocol implementation]
> (let [[protocol-name signature] protocol]
> (eval `(defprotocol ~protocol-name ~signature))
> `(fn [] (reify ~protocol-name ~implementation))))
>
> Using eval doesn’t feel right though.
>
> I guess I could modify my code to avoid using protocols, but it seemed to
> me to be the most natural way of achieving what I wanted.
>
> I was just wondering if anybody had any opinion or suggestion about that.
> Am I going off track? Is there a more idiomatic way of doing things that I
> missed?
>
> Thanks,
> Vincent
>
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