protocol-ize with-open

Question

Good use (and documentation example) of protocols: make with-open aware of a Closable protocol for APIs that use a different close convention. See http://groups.google.com/group/clojure/browse_thread/thread/86c87e1fc4b1347c

Answer 1

Comment made by: importer

Converted from http://www.assembla.com/spaces/clojure/tickets/308

Answer 2

Comment made by: tsdh

Added a CloseableResource protocol and extended it on java.io.Closeable (implemented by all Readers, Writers, Streams, Channels, Sockets). Use it in with-open.

All tests pass.

Answer 3

Comment made by: tsdh

Seems to be related to Scopes (http://dev.clojure.org/jira/browse/CLJ-2).

Answer 4

Comment made by: tsdh

Updated patch.

Answer 5

Comment made by: jafingerhut

Patch 0001-Added-ClosableResource-protocol-for-with-open.patch dated 08/Mar/12 applies, builds, and tests cleanly on latest master as of Apr 2 2012. Tassilo has signed a CA.

Answer 6

Comment made by: tsdh

Updated patch to apply cleanly against master again.

Answer 7

Comment made by: bbloom

I looked up this ticket because I ran in to a reflection warning: with-open does not hint it's binding with java.io.Closeable

Some feedback on the patch:

1) This is a breaking change for anyone relying on the close method to be duck-typed.

2) CloseableResource is a bit long. clojure.core.protocols.Closeable is plenty unambiguous.

3) Rather than extending CloseableResource to java.io.Closeable, you can use the little known (undocumented? unsupported?) :on-interface directive:

(defprotocol Closeable :on-interface java.io.Closeable (close [this]))

That would perform much better than the existing patch.

Answer 8

Comment made by: tsdh

Hi Brandon, two questions:

Could 1) be circumvented somehow by providing a default implementation somehow? I guess the protocol could be extended upon Object with implementation (.close this), but that would give a reflection warning since Object has no close method. Probably one could extend upon Object and in the implementation search a "close" method using java.lang.reflect and throw an exception if none could be found?

Could you please tell me a bit more about the :on-interface option? How does that differ from extend? And how do I add the implementation, i.e., (.close this) with that option?

Answer 9

Reference: https://clojure.atlassian.net/browse/CLJ-308 (reported by alex+import)