Clojure Q&A - Recent questions tagged proxy

Using proxy to extend MemoryStream fails on Close

Sun, 15 Jan 2023 00:08:50 +0000

For some unit tests, I need to be able to read the contents of a MemoryStream after it has been closed. As this isn't possible, I want to create a memory stream proxy where the call to Close is a no-op, allowing me to read from it even when code outside of my control has attemped closing it. This however won't work.

REPL

(import [System.IO MemoryStream StreamWriter])
(let [stream (proxy [MemoryStream] [] (Close [] (println "close!")))]
  (doto (StreamWriter. stream)
        (.Write "testing")
        (.Close)))

output

close!
Execution error (InvalidProgramException) at System.Diagnostics.StackFrame/Write (NO_FILE:0).
Cannot create boxed ByRef-like values.

While it's evident my proxied Close method is called, an error is thrown just after.

using `proxy` might make an otherwise additive change breaking

Tue, 30 Mar 2021 10:33:11 +0000

Given an interface

public interface Foo {
    void bar(Object o);
}

And a proxy over that interface:

(proxy [Foo] []
  (bar [o] (baz o)))

When the interface gets updated to

public interface Foo {
  void bar(Object o);

  // java 8 default method, quote from 
  // https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html
  // "Default methods enable you to add new functionality to the
  // interfaces of your libraries and ensure binary compatibility
  // with code written for older versions of those interfaces."
  default void bar(Object o, Object p) {
    bar(o);
  }
}

Calls to instanceCreatedByProxy.bar(o, p) will not work as it could be expected.

reify is not affected by this.

Repro repo: https://github.com/imrekoszo/proxy-default

There is no indication of this behavior in the docstring of proxy, the only reference I saw to this is an example on clojuredocs.

This is troublesome as it might prevent an otherwise non-breaking library upgrade. If the code calling proxy is under one's control they could use reify instead (especially as this is recommended in the official docs). However if it is in an external library this is not a viable workaround.

Is this a bug or a known limitation of proxy?
If the latter, then could this perhaps be made more visible, for example through a warning in the docstring?

Edit:
- link to original slack thread
- added link to reify recommendation

Implementing a type or record to support defmethod in CLJ

Fri, 17 Jan 2020 18:37:33 +0000

Thanks to IMultiFn in CLJS types and records can be easily extended to behave as multimethods. For example:

`(defrecord MyMulti
[add-method remove-method dispatch-fn method-table]
IMultiFn
(-add-method [_ dispatch-val f]

(add-method dispatch-val f))

(-remove-method [_ dispatch-val]

(remove-method dispatch-val))

(-methods [_] @method-table))

(def my-multi (->MyMulti ...))

(defmethod my-multi :foo
[x]
...)
`

In Clojure however this isn't as easy because multimethods are not implemented using protocols. IS there ANY way to make a record or type to work with defmethod in CLJ?

Thanks!