Clojure

• Type: Defect
• Status: Closed
• Resolution: Completed
• Affects Version/s: None
• Fix Version/s: Release 1.2
• Component/s: None
• Labels:

  None

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]))
</code></pre>

and file protoproblem/impl.clj:
<pre><code>   (ns protoproblem.impl
     (:require [protoproblem.proto :as proto]))

   (deftype ArraySteps [a]
     proto/Steps
     (one [x] (first a))
     (two [x] (second a)))
</code></pre>

In the repl, everything works as expected:
<pre><code>   user> (load "protoproblem/proto")
   nil
   user> (load "protoproblem/impl")
   nil
   user> (protoproblem.proto/one (protoproblem.impl.ArraySteps. [1 2]))
   1
</code></pre>

Now, we redefine the protocol by commenting out the 'one' method def in proto.clj, and then reloading it:
<pre><code>   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)
</code></pre>

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:
<pre><code>   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.)

When using "reify" inside a factory function instead of the constructor for a "deftype", everything works as expected.

• Assignee:

  Rich Hickey

  Reporter:

  Anonymous

• Created:

  20/May/10 10:53 AM

  Updated:

  24/Aug/10 9:12 AM

  Resolved:

  24/Aug/10 9:12 AM