At the moment assert macro accepts only a single argument - a test. If
the test fails (is false), an exception is thrown. But the only
information available (until we have better introspection tools) is
the test _expression_. This causes problems when working on many items
that should satisfy an assertion, but the assertion expression is the
same for all of them (like, when iterating through a sequence) - it is
impossible to see for which items exactly the assertion fails.
For this reason I think assert macro should accept another parameter -
something to construct a meaningful assertion failure reason. A
simplistic approach would be to just pass a string that has been
constructed using str function. But this would involve too much
unnecessary computation/consing since assertions are usually expected
not to fail (in which case the failure message is not needed and
should not be constructed). So my proposed (below) accepts a second
parameter, which should be a form that results in a string. It could
also accept variable number of parameters and implicitly call str on
them, but that I think is a bad style.
Comments welcome. And I hope some variation of it makes it to the
clojure.core.
(defmacro assert
"Evaluates expr and throws an exception if it does not evaluate to
logical true. message, if present, should be a an expression that
evaluates to
string describing the assertion failure."
([x]
`(when-not ~x
(throw (new Exception (str "Assert failed: " (pr-str '~x))))))
([x message]
`(let [message# (fn [] ~message)]
(when-not ~x
(throw (new Exception (str "Assert failed:
" (message#))))))))
--
Jānis Džeriņš
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