While we are talking about assert: I recently wanted for assert to return the value of the expression, so I could embed asserts inside a Fact test and get detailed reporting about what failed.
I checked a few other functional languages and none of their asserts return a value either. This surprised me. Should assert return its value? If not, what should I name the function that works just like assert but returns its value? Stuart > 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 > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
