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"))))
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