I was in to process of writing a rant against condp when I realized I had been thinking about it all wrong. Here is what I wanted at first:
(defmacro casep [pred & clauses] `(condp #(%2 %1) ~pred ~...@clauses)) In other words, I want to be able to specify a unary predicate instead of a binary predicate and a single argument for it. I know from reading discussions on clojure-dev that the main advantage of condp over casep is that condp evaluates its first two arguments only once, and writing the same code using casep would could cause a lot of redundant evaluation. While I agree that condp earns its keep on that basis, I was still annoyed by the way condp calls the predicate you give it. I was hoping that condp could use an off-the-shelf function like apply, so I could get the effect of casep just by writing code like (condp apply my-unary-predicate ...); IMHO this is a perfectly readable idiom, but, alas, it's wrong; in place of apply, you have to use something like #(%2 %1), which looks far too Perlish for my taste. The saving grace of condp is that there are a lot more useful binary predicates than I had realized at first. When I went back and re-examined some of my code, I found you can do some cool things with condp that would not be possible if condp passed the arguments to the predicate in a different order: ;; Which value is less then 3? (condp < 3 x ... y ... z ...) ;; Which set contains x? (condp contains? x set-a ... set-b ... set-c ...) ;; Which object (represented as a map) is flagged as special? (condp get :special? map-a ... map-b ... map-c) ;; Which predicate does x satisfy? (condp apply [x] integer? ... string? ... nil? ...) Even a use case that's tailor-made for casep is not so bad if you're willing to turn the test expressions into functions: ;; Which item satisfies my-pred? (condp apply [my-pred] #(% item-a) ... #(% item-b) ... #(% item-c) ...) -- 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
