There is no specific function for this, however you can dynamically rebind the var holding the stream that println writes to like this: (println “Hello, STDOUT!”) (binding [*out* *err*] (println “Hello, STDERR!”)) In my REPL, the colour indicates the stream (red is STDERR):
I find alter-var-root very rarely comes up in idiomatic Clojure code; not that there is anything wrong with it, it’s just intended for corner cases. If you find yourself using it to build loops and such it’s a sign something needs a different approach. I mostly see it in initialization routines for setting access credentials … Read more
Like any Lisp dialect, Clojure draws a very fuzzy line between API and DSL and therefore the term doesn’t hold the same mystique that it does in other languages. Lisp programmers tend to write their programs as layers of DSLs, each layer serving those above it. Having said that, here are a few that you … Read more
(:color my-car) is fairly standard. There are a few reasons for this, and I won’t go into all of them. But here’s an example. Because :color is a constant, and my-car is not, hotspot can completely inline the dynamic dispatch of color.invoke(m), which it can’t do with m.invoke(color) (in some java pseudo-code). That gets even … Read more
(set! *unchecked-math* true) (defn add-up ^long [^long n] (loop [n n i 0 sum 0] (if (< n i) sum (recur n (inc i) (+ i sum))))) (defn fib ^long [^long n] (if (<= n 1) 1 (+ (fib (dec n)) (fib (- n 2))))) (comment ;; ~130ms (dotimes [_ 10] (time (add-up 1e8))) ;; … Read more
If you’re going to use: #(first (reverse %)) You might as well compose the function with “comp”: (comp first reverse) This is probably a little more idiomatic and easier to read. The same caveat about “reverse” not being lazy applies here.
Here’s another way: (defn factorial [n] (reduce * (range 1 (inc n)))) This won’t blow the stack because range returns a lazy seq, and reduce walks across the seq without holding onto the head. reduce makes use of chunked seqs if it can, so this can actually perform better than using recur yourself. Using Siddhartha … Read more
Clojure Vars can have thread-local bindings. binding uses these, while with-redefs actually alters the root binding (which is someting like the default value) of the var. Another difference is that binding only works for :dynamic vars while with-redefs works for all vars. Examples: user=> (def ^:dynamic *a* 1) #’user/*a* user=> (binding [*a* 2] *a*) 2 … Read more
Dynamic dispatch and namespace resolution are two different things. In many object systems classes are also used for namespaces. Also note that often both the class and the namespace are tied to a file. So these object systems conflate at least three things: class definitions with their slots and methods the namespace for identifiers the … Read more
concat returns a lazy sequence. user=> (doc concat) ————————- clojure.core/concat ([] [x] [x y] [x y & zs]) Returns a lazy seq representing the concatenation of the elements in the supplied colls. you can convert it back to a vector with into: user=> (into [] (concat [1 2] [3 4] [5 6])) [1 2 3 … Read more