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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