Support (try .. (catch :default _ ..)) for portable "catch-all"

Question

Clojurescript has this to expose the untyped catch, which is equivalent to (catch Throwable _) on java.

http://dev.clojure.org/jira/browse/CLJS-661

Proposals

1) add (catch :default ) to mean (catch Throwable )
2) add (catch :default ) to mean (catch Exception )
3) add (catch :all ) to mean (catch Throwable )

Please see design page for discussion of proposals: http://dev.clojure.org/display/design/Platform Errors

Patches

v001 implements just 1)

This patch is more permissive than my patch for CLJS: The CLJS patch ensures :default catch blocks occur between non-default catch blocks and finally blocks, if present. This patch just makes (catch :default ...) a synonym for (catch Throwable ...). I wanted to keep the change to the compiler minimum.

Open Question: Catch Throwable (patch v001 does this) or Exception? Alternatively, a more carefully crafted list of "non-fatal" errors. See Scala's NonFatal pattern extractor: http://www.scala-lang.org/api/current/scala/util/control/NonFatal$.html

v002 implements 2) + 3)

This builds on v001, so the same caveat about clause ordering applies.

v003 implements just 2)

This builds on v001, so the same caveat about clause ordering applies.

Answer 1

Reference: https://clojure.atlassian.net/browse/CLJ-1293 (reported by bbloom)

Answer 2

Comment made by: bbloom

Noticed this switched from "Minor" to "Critical", so I figured I should mention that I later realized that we might want :default to catch Exception instead of Throwable, so as to avoid catching Error subclasses. Javadocs say: "An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch." If that's what we actually want, I can provide an updated patch.

Answer 3

Comment made by: alexmiller

Seems like an open question, might be best just to list it as such in the description.

I don't really expect to reach consensus on the ticket or patch right now, just trying to update priorities and raise visibility for discussion with Rich once we get to 1.8.

Answer 4

Comment made by: bendlas

I'm in favor of catching Exception. It is the :default on java (as stated in the docs), so catching Throwable is a platform-specific thing to do and it would still be possible.

Answer 5

Comment made by: bendlas

Hm, realizing now, that my last comment is at odds with the design discussion about being able to catch anything in javascript.

Attached patch v002 implements :all in addition to :default.

Answer 6

Comment made by: bendlas

Realized, that catch-all vs catch-Exception is only a shallow contradiction: (catch Exception _) is - for all intents and purposes - the catch-all of java. Since the catch-absolutely-all is accessible in java through a regular catch, the driving need in clojurescript doesn't apply to clojure. The driving need in clojure is portability. In clojurescript, this is conflated with exposing an otherwise inaccessible platform feature, but that needs to not drive the general design.

Attached v003