On Mon, Jan 24, 2011 at 7:32 PM, Laurent PETIT <laurent.pe...@gmail.com> wrote:
> 2011/1/25 Ken Wesson <kwess...@gmail.com>:
>> Are we generally agreed that we want to preserve the body exception,
>> if any, else the close exception, if any, else the body return value?
>>
>> Then, without further ado:
>>
>> (defmacro with-open2 [binding-exprs & body]
>> (if (> (count binding-exprs) 2)
>> `(with-open2 ~(vec (take 2 binding-exprs))
>> (with-open2 ~(vec (drop 2 binding-exprs))
>> ~@body))
>> (let [[sym expr] binding-exprs]
>> `(let [~sym ~expr
>> res# (try
>> [(do ~@body)]
>> (catch Throwable t# t#))]
>> (if (vector? res#) ; body didn't throw
>> (do
>> (.close ~sym) ; so let close throw
>> (res# 0))
>> (do ; body threw
>> (try (.close ~sym) (catch Throwable _#)) ; eat close exception
>> (throw res#))))))) ; and rethrow body exception
>>
>> user=> (with-open [x (Foo.)] (doit x))
>> #<CompilerException java.lang.Exception: close exception
>> (NO_SOURCE_FILE:242)>
>> user=> (with-open2 [x (Foo.)] (doit x))
>> #<CompilerException java.lang.Exception: doit exception (NO_SOURCE_FILE:40)>
>
> What about starting from the current definition of with-open in
> clojure.core, improving upon it (and at the same time, not building
> intermediate wrapper vector) :
>
> (in-ns 'clojure.core)
> (defmacro with-open
> "bindings => [name init ...]
>
> Evaluates body in a try expression with names bound to the values
> of the inits, and a finally clause that calls (.close name) on each
> name in reverse order."
> {:added "1.0"}
> [bindings & body]
> (assert-args with-open
> (vector? bindings) "a vector for its binding"
> (even? (count bindings)) "an even number of forms in binding vector")
> (cond
> (= (count bindings) 0) `(do ~@body)
> (symbol? (bindings 0)) `(let ~(subvec bindings 0 2)
> (try
> (let [res# (with-open ~(subvec
> bindings 2) ~@body)]
> ;body didn't throw
> (. ~(bindings 0) close)
> res#)
> (catch Throwable t#
> (try (. ~(bindings 0) close) (catch
> Throwable _)) ; eat close exception
> (throw t#)))) ; and rethrow body exception
> :else (throw (IllegalArgumentException.
> "with-open only allows Symbols in bindings"))))
Ah, I guess it may be a bit cleaner to just eschew "finally"
altogether and close in the try and in the catch. The one thing that's
a bit icky about that is that if the body close throws, the catch
clause tries to close the stream again. On the other hand, my
version's vector wrap is a bit icky, perhaps. :)
Yours works if the binding vector is empty, too; I suppose that's a
good idea. You've also incorporated the core version's error reporting
code. If this makes it into core in any form it should probably
include those features. :)
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