Sometimes I want to do the same thing for multiple classes of
exceptions without writing very similar catch clauses repeatedly, so I
wrote a macro:
(defmacro try*
"Exactly like try, except that catch statements can contain a
sequence of exception-types which should all be handled in the same
way. E.g.:
(try* expr1 expr2
(catch ArithmeticException a-exc ...)
(catch [Exception AssertionError] e ...)
(finally ...)"
[& exprs]
(letfn [(is-multi-catch? [expr] (sequential? (second expr)))
(expand-catches [expr] (map (fn [exn-type] (list* 'catch
exn-type (drop 2 expr))) (second expr)))
(is-postlude? [expr] (and (list? expr) (#{'finally 'catch}
(first expr))))]
(let [[exprs postlude] (split-with (complement is-postlude?) exprs)
[catches finally] (split-with (comp #{'catch} first) postlude)
catches (mapcat #(if (is-multi-catch? %) (expand-catches %)
[%]) catches)]
;; kick errors down to core try
`(try ~@exprs
~@catches
~@finally))))
This seems to work fine.
But it *doesn't* work inside a different macro:
user> (defmacro g [& forms] `(try* ~@forms (catch [Exception
AssertionError] e# 6)))
#'user/g
user> (g 2 3)
; Evaluation aborted
with the error: Unable to resolve classname: [java.lang.Exception
java.lang.AssertionError]
These macroexpansions are as expected:
user> (macroexpand-1 '(g 2 3))
(user/try* 2 3 (catch [java.lang.Exception java.lang.AssertionError]
e__45530__auto__ 6))
user> (macroexpand-1 '(user/try* 2 3 (catch [java.lang.Exception
java.lang.AssertionError] e__45530__auto__ 6)))
(try 2 3 (catch java.lang.Exception e__45530__auto__ 6) (catch
java.lang.AssertionError e__45530__auto__ 6))
But the second one here (the first is identical to the above) is not:
user> (macroexpand-1 '(g 2 3))
(user/try* 2 3 (catch [java.lang.Exception java.lang.AssertionError]
e__45530__auto__ 6))
user> (macroexpand-1 *1)
(try 2 3 (catch [java.lang.Exception java.lang.AssertionError]
e__45530__auto__ 6))
What's going on? Where'd my try* go?
--
Ben Wolfson
"Human kind has used its intelligence to vary the flavour of drinks,
which may be sweet, aromatic, fermented or spirit-based. ... Family
and social life also offer numerous other occasions to consume drinks
for pleasure." [Larousse, "Drink" entry]
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