I'm loving protocols, but I keep having to restart my JVM to avoid a puzzling
issue when changing protocol definitions.
Here's the simplest way to reproduce the problem:
Start with file protoproblem/proto.clj:
(ns protoproblem.proto)
(defprotocol Steps
(one [x])
(two [x]))
and file protoproblem/impl.clj:
(ns protoproblem.impl
(:require [protoproblem.proto :as proto]))
(deftype ArraySteps [a]
proto/Steps
(one [x] (first a))
(two [x] (second a)))
In the repl, everything works as expected:
user> (load "protoproblem/proto")
nil
user> (load "protoproblem/impl")
nil
user> (protoproblem.proto/one (protoproblem.impl.ArraySteps. [1 2]))
1
Now, we redefine the protocol by commenting out the 'one' method def in
proto.clj, and then reloading it:
user=> (load "protoproblem/proto")
nil
user=> (protoproblem.proto/one (protoproblem.impl.ArraySteps. [1 2]))
java.lang.NullPointerException (NO_SOURCE_FILE:9)
user=> (load "protoproblem/impl")
java.lang.IllegalArgumentException: Can't define method not in interfaces:
one (impl.clj:5)
That's pretty much as expected. But what if we try to get back to how things
were, by reversing the changes in proto.clj, and reloading? This is where the
problems arise:
user=> (load "protoproblem/proto")
nil
user=> (load "protoproblem/impl")
nil
user=> (protoproblem.proto/one (protoproblem.impl.ArraySteps. [1 2]))
java.lang.IllegalArgumentException: No implementation of method: :one of
protocol: #'protoproblem.proto/Steps found for class:
protoproblem.impl.ArraySteps (NO_SOURCE_FILE:0)
Ouch!
At this point, I have to restart my JVM to proceed, which is making incremental
development of protocols-based code unusually tedious.
I initially encountered the behaviour using "C-c C-k" under
slime/swank-clojure, but the above transcript is from a plain command-line
repl, using a Clojure 1.2 snapshot from today. (The same thing happened with a
snapshot from 2 weeks ago.)
Is this a classloader issue, or something else? Any help greatly appreciated.
-Steve
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