[CLJ-1061] when-first double evaluation

Sat, 20 Oct 2012 09:53:42 -0500

The when-first macro will evaluate the xs expression twice. Admittedly, it does exactly what the doc string says, but that seems undesirable to me. Even without side effects, there's a potential performance issue if xs is some expensive operation.

Patch attached. The main diff is:

- `(when (seq ~xs)
- (let [~x (first ~xs)]
- ~@body))))
+ `(when-let xs# (seq ~xs)
+ (let ~x (first xs#)
+ ~@body))))

Clojure JIRA

[CLJ-1061] when-first double evaluation