On Wed, Mar 28, 2012 at 2:13 PM, Timothy Baldridge <tbaldri...@gmail.com> wrote: >>>is there a use case behind apply being lazy when Clojure is otherwise a >>>strictly evaluating language > > In clojure-py we have to pass vararg arguments as tuples. So it ends > up a lot like > > (to-tuple (concat args seqarg)) > > I always saw the seq argument in IFn as a crutch to get around the "we > have too many vars to pass to this function" problem. Basically, since > the JVM doesn't support lazy varargs, Clojure doesn't either.
Apply isn't really non-strict. If your source contains (apply foo x y z w) then apply, foo, x, y, z, and w will all be evaluated, in left-to-right order, before foo is called with x, y, z, and the contents of w as arguments. If w is a literal such as [w1 w2 w3 w4] then w1, w2, w3, and w4 will get evaluated, after z and in left-to-right order, before foo is called. In other words, any side effects in a line like (apply foo x y z [w1 w2 w3 w4]) happen in the order one would expect. And if w is really a lazy sequence, and a lambda involved in generating it has side effects, the fact that the side effects are in a lambda (or not even in the (apply...) line) tells you that the side effects may be delayed. Further, it's possible to get a kind of non-strict-analogous behavior (side effects may be delayed, reordered, or not happen at all) in Clojure if foo and its caller are written specially. For example, (defn weird-and [x y] (if @x @y)) (weird-and (delay (long-running-test-1)) (delay (long-running-test-2))) will only compute the second expensive test if the first didn't return logical false. (The built-in and macro also short-circuits, but does so by being a macro. Unlike and, weird-and could be passed to a HOF. But the HOF would have to know to pass it delays rather than normal booleans. You could do (reduce weird-and [(delay this) (delay that) ...]).) -- 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 Note that posts from new members are moderated - please be patient with your first post. 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
