Can you include an example usage of this function? Thanks. On Wed, Dec 3, 2008 at 12:51 PM, Meikel Brandmeyer <[EMAIL PROTECTED]> wrote:
> Hi Stuart and Rich, > > Am 03.12.2008 um 19:00 schrieb Stuart Sierra: > >> I'm pretty sure I don't like the sound of that at all. We had a nice >>> discussion about fcase/condf, which I'd like to get in, here: >>> >>> http://groups.google.com/group/clojure/browse_frm/thread/dee910bef629... >>> >> >> And I haven't forgotten about that, just haven't had time to work on >> it. Anyone else who wants to tackle it is welcome. >> > > How about the following: > > (defmacro condp > "condp compares the given needle with the first clause using the given > predicate. In case the predicate returns true the second clause is > returned. Otherwise condp goes on with the rest of the clauses. In > case there is an odd number of clauses the last one will be returned > if no preceding clause matched. If no default clause is given an > exception is thrown. The predicate is called with needle as first > argument and the first clause as second argument." > [pred needle & clauses] > (let [c (count clauses) > par (rem c 2) > cls (take (if (zero? par) c (dec c)) clauses) > cls (mapcat (fn [[x c]] `[(~xprd ~xndl ~x) ~c]) (partition 2 cls)) > xndl (gensym "condp_needle__") > xprd (gensym "condp_predicate__") > lst (if (zero? par) > `(throw (Exception. (str "No condp clause matched for: " > (prn-str ~xndl)))) > (last clauses))] > `(let [~xprd ~pred > ~xndl ~needle] > (cond > [EMAIL PROTECTED] > :else ~lst)))) > > I mulled about the (x _) syntax, but to be honest: I think #() is > perfectly sufficient. We would need to quote all the contents to > prevent multiple evaluation, but then we have to recurse and check > for _ vs. non-_... > > I expect the predicate to be a function of two arguments with the first > being the needle and the second being the clause, we compare to. > Predicates where order doesn't matter or the order agrees may be simply > used. Others or more complicated expressions can be wrapped in a #(). > Here we still have a problem for #(pred (complicated-computation) %2 > %1).... > > In case the number of clauses is even, and no clause matched the > predicate, we throw an exception. In case the number of clauses is > odd, we return the last one as default. I think adding a simple nil > is tolerable, making the "I ignore a failed run" explicit. > > What do you think? > > Sincerely > Meikel > > > > --~--~---------~--~----~------------~-------~--~----~ 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 [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
