Ah, :else is an unfortunate choice. The cond macro keeps testing clauses until it finds one that is true. :else was chose because it is simply not nil, and therefor always true.
I would re-write your fn like so: (defn sign [x] (cond (> x 0) "Positive" (< x 0) "Negative" true "Zero")) Sean On Dec 19, 10:22 pm, Mike K <mbk.li...@gmail.com> wrote: > What, exactly, is a "clause" in clojure? From seeing the term used in > context, I inferred that it meant something along the lines of "a > keyword which can optionally be used to change the semantics of a > form". Apparently, it means more than that. > > This is valid clojure: > > (defn sign [x] (cond (> x 0) "Positive" (< x 0) "Negative" :else > "Zero")) > > I had no idea that I could use :else for the fallthrough case of a > "cond", so I looked it up in the docs: > > user> (doc cond) > ------------------------- > clojure.core/cond > ([& clauses]) > Macro > Takes a set of test/expr pairs. It evaluates each test one at a > time. If a test returns logical true, cond evaluates and returns > the value of the corresponding expr and doesn't evaluate any of the > other tests or exprs. (cond) returns nil. > > Apparently, each "test/expr pair" is a clause, or perhaps each test > and each expression is a clause. No mention of supporting an :else > "clause" though. Let's check out the source code: > > (defmacro cond > "Takes a set of test/expr pairs. It evaluates each test one at a > time. If a test returns logical true, cond evaluates and returns > the value of the corresponding expr and doesn't evaluate any of the > other tests or exprs. (cond) returns nil." > [& clauses] > (when clauses > (list 'if (first clauses) > (if (next clauses) > (second clauses) > (throw (IllegalArgumentException. > "cond requires an even number of forms"))) > (cons 'clojure.core/cond (next (next clauses)))))) > > I don't know much about macros, but it is clear that "cond" builds on > "if". However, the documentation for "if" mentions nothing > about :else, and the source for "if" is not available, presumably > because it's implemented in java. > > Is there a way to have discovered the existence of :else from the > documentation alone? Is this just an oversight in the docs, or is > there a more general point concerning clauses that I'm missing? > > Mike -- 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
