Thanks, Evan; I had a use-case where the truthiness of nil would have forced me out of `let-else.' This new predicate-abstraction is beautiful.
Quoth Evan Gamble on Sweetmorn, the 49th of The Aftermath: > Thanks for your comment, Sam. > > Before you posted the comment, Peter Danenberg had asked if I would > modify let-else to include the behavior of your let? macro. Your > comment and his request have spurred me to action. > > I've modified the macro to accept optional :when <pred> and :else > <expr> clauses after bindings. The :when clause acts just like > your :ensure clause. > It works with just :when or just :else or both, in either order. > > I've also renamed my macro to be let?, because with the addition > of :when, the name let-else doesn't make sense. > > The jar is still at https://clojars.org/org.clojars.egamble/let-else > The code is still at https://github.com/egamble/let-else > > - Evan > > On Dec 6, 7:13 pm, Sam Ritchie <sritchi...@gmail.com> wrote: > > I had a pattern that kept popping up in code of: > > > > (let [x (foo) > > y (bar)] > > (when y > > (let [ ....] > > ....))) > > > > that check jarred me, so I put this > > together:https://gist.github.com/1347312. On reflection, discomfort with > > indentation > > levels probably isn't near the top of the "to macro or not to macro?" > > checklist. > > > > (defmacro let? > > [bindings & body] > > (let [[bind [kwd pred & more]] (split-with (complement #{:ensure}) > > bindings)] > > `(let [~@bind] > > ~@(cond (and kwd more) [`(when ~pred (check-let [~@more] ~@body))] > > kwd [`(when ~pred ~@body)] > > :else body)))) > > > > (let? [x 100 > > y 300 > > :ensure (pos? y) > > z (- y 250)] > > z) > > > > ;; expands to > > > > (let [x 100 > > y 300] > > (when (pos? y) > > (let [z (- y 250)] > > z))) ;; => 50 > > > > ;; and returns 50. The following returns nil: > > > > (let? [x 100 > > > > y 300 > > > > :ensure (neg? y) > > > > z (- y 250)] > > z) ;; => nil > > > > > > > > > > > > > > > > > > > > On Tue, Dec 6, 2011 at 11:51 AM, Evan Gamble <solar.f...@gmail.com> wrote: > > > I noticed in my code that I often nest a let inside an if-let, or vice- > > > versa, so I wrote a macro let-else that expands into nested lets, > > > except where there's an :else <expr> after a binding, in which case > > > that binding expands into an if-let. > > > > > E.g. > > > > > (let-else > > > [foo (f1) :else (e) > > > bar (f2)] > > > (b1) > > > (b2)) > > > > > expands into > > > > > (if-let [foo (f1)] > > > (let [bar (f2)] > > > (b1) > > > (b2)) > > > (e)) > > > > > The jar is athttps://clojars.org/org.clojars.egamble/let-else > > > The code is athttps://github.com/egamble/let-else > > > > > - Evan > > > > > -- > > > 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 > > > > -- > > Sam Ritchie, Twitter Inc > > 703.662.1337 > > @sritchie09 > > > > (Too brief? Here's why!http://emailcharter.org) > > -- > 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 > -- 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
