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  • Understanding Clojure concurrency example

    - by dusha
    Hello, I just go through various documentation on Clojure concurrency and came accross the example on the website (http://clojure.org/concurrent_programming). (import '(java.util.concurrent Executors)) (defn test-stm [nitems nthreads niters] (let [refs (map ref (replicate nitems 0)) pool (Executors/newFixedThreadPool nthreads) tasks (map (fn [t] (fn [] (dotimes [n niters] (dosync (doseq [r refs] (alter r + 1 t)))))) (range nthreads))] (doseq [future (.invokeAll pool tasks)] (.get future)) (.shutdown pool) (map deref refs))) I understand what it does and how it works, but I don't get why the second anonymous function fn[] is needed? Many thanks, dusha. P.S. Without this second fn [] I get NullPointerException.

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  • Adding fields to a proxied class in Clojure

    - by mikera
    I'm using "proxy" to extend various Swing classes in a Clojure GUI application, generally with code that looks something like: (def ^JPanel mypanel (proxy [JPanel] [] (paintComponent [#^Graphics g] (.drawImage g background-image 0 0 nil)))) This works well but I can't figure out how to add additional fields to the newly extended class, for example making the background-image a field that could be subsequently updated. This would be pretty easy and common practice in Java. Is there a good way to do this in Clojure? Or is there another preferred method to achieve the same effect?

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  • how is a macro expanded in clojure?

    - by john wang
    In the book Programming Clojure(Stuart), when read how macros are expanded I got confused. user=> (defmacro chain ([x form] (list '. x form)) ([x form & more] (concat (list 'chain (list '. x form)) more))) #'user/chain The above macro can be expanded as: user=> (macroexpand '(chain a b c)) (. (. a b) c) But the following is only expanded to the first level: user=> (macroexpand '(and a b c)) (let* [and__3822__auto__ a] (if and__3822__auto__ (clojure.core/and b c) and__3822__auto__)) The and macro source: user=> (source and) (defmacro and([] true) ([x] x) ([x & next] `(let [and# ~x] (if and# (and ~@next) and#)))) Why is the chain macro expanded all the way but the and not ? Why is it not expanded to something like the following: user=> (macroexpand '(chain a b c d)) (. (chain a b c) d)

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  • Organizing test hierarchy in clojure project

    - by Sergey
    There are two directories in a clojure project - src/ and test/. There's a file my_methods.clj in the src/calc/ directory which starts with (ns calc.my_methods...). I want to create a test file for it in test directory - test/my_methods-test.clj (ns test.my_methods-test (:require [calc.my_methods]) (:use clojure.test)) In the $CLASSPATH there are both project root directory and src/ directory. But the exception is still "Could not locate calc/my_methods__init.class or calc/my_methods.clj on classpath". What is the problem with requiring it in the test file? echo $CLASSPATH gives this: ~/project:~/project/src

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  • Difference in F# and Clojure when calling redefined functions

    - by Michiel Borkent
    In F#: > let f x = x + 2;; val f : int -> int > let g x = f x;; val g : int -> int > g 10;; val it : int = 12 > let f x = x + 3;; val f : int -> int > g 10;; val it : int = 12 In Clojure: 1:1 user=> (defn f [x] (+ x 2)) #'user/f 1:2 user=> (defn g [x] (f x)) #'user/g 1:3 user=> (g 10) 12 1:4 user=> (defn f [x] (+ x 3)) #'user/f 1:5 user=> (g 10) 13 Note that in Clojure the most recent version of f gets called in the last line. In F# however still the old version of f is called. Why is this and how does this work?

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  • Database Functional Programming in Clojure

    - by Ralph
    "It is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail." - Abraham Maslow I need to write a tool to dump a large hierarchical (SQL) database to XML. The hierarchy consists of a Person table with subsidiary Address, Phone, etc. tables. I have to dump thousands of rows, so I would like to do so incrementally and not keep the whole XML file in memory. I would like to isolate non-pure function code to a small portion of the application. I am thinking that this might be a good opportunity to explore FP and concurrency in Clojure. I can also show the benefits of immutable data and multi-core utilization to my skeptical co-workers. I'm not sure how the overall architecture of the application should be. I am thinking that I can use an impure function to retrieve the database rows and return a lazy sequence that can then be processed by a pure function that returns an XML fragment. For each Person row, I can create a Future and have several processed in parallel (the output order does not matter). As each Person is processed, the task will retrieve the appropriate rows from the Address, Phone, etc. tables and generate the nested XML. I can use a a generic function to process most of the tables, relying on database meta-data to get the column information, with special functions for the few tables that need custom processing. These functions could be listed in a map(table name -> function). Am I going about this in the right way? I can easily fall back to doing it in OO using Java, but that would be no fun. BTW, are there any good books on FP patterns or architecture? I have several good books on Clojure, Scala, and F#, but although each covers the language well, none look at the "big picture" of function programming design.

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  • Difference in calling redefined functions in F# and Clojure

    - by Michiel Borkent
    In F#: > let f x = x + 2;; val f : int -> int > let g x = f x;; val g : int -> int > g 10;; val it : int = 12 > let f x = x + 3;; val f : int -> int > g 10;; val it : int = 12 In Clojure: (defn f [x] (+ x 2)) (defn g [x] (f x)) (g 10) ;; => 12 (defn f [x] (+ x 3)) (g 10) ;; => 13 Note that in Clojure the most recent version of f gets called in the last line. In F# however still the old version of f is called. Why is this?

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  • Getting the name of a Clojure struct type?

    - by j-g-faustus
    When defining a struct type and instance, I can print the value and get the "struct" implementation type: (defstruct person :name :age) (def p (struct person "peter" 30)) user=> p {:name "peter", :age 30} user=> (type p) clojure.lang.PersistentStructMap But is it possible to tell whether p is an instance of the struct type "person"?

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  • How to go about reading a web page lazily in Clojure

    - by Rayne
    I and a friend recently implemented link grabbing in my Clojure IRC bot. When it sees a link, it slurp*s the page and grabs the title from the page. The problem is that it has to slurp* the ENTIRE page just to grab the link. How does one go about reading a page lazily until the first ?

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  • How do I stop jetty server in clojure?

    - by Mad Wombat
    I am writing a web application using ring and clojure. I am using the jetty adapter for the development server and emacs/SLIME for IDE. While wrap-reload does help, run-jetty blocks my slime session and I would like to be able to start/stop it at will without having to run it in a separate terminal session. Ideally, I would like to define a server agent and functions start-server and stop-server that would start/stop the server inside the agent. Is this possible?

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  • Whats the point of lazy-seq in clojure?

    - by dbyrne
    I am looking through some example Fibonacci sequence clojure code: (def fibs (lazy-cat [1 2] (map + fibs (rest fibs)))) I generally understand what is going on, but don't quite understand the point of lazy-cat. I know that lazy-cat is a macro that is translating to something like this: (def fibs (concat (lazy-seq [1 2]) (lazy-seq (map + fibs (rest fibs))))) What exactly is lazy-seq accomplishing? It would still be evaluated lazily even without lazy-seq? Is this strictly for caching purposes?

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  • Clojure program reading its own MANIFEST.MF

    - by Ralph
    How can a Clojure program find its own MANIFEST.MF (assuming it is packaged in a JAR file). I am trying to do this from my "-main" function, but I can't find a class to use in the following code: (.getValue (.. (java.util.jar.Manifest. (.openStream (java.net.URL. (str "jar:" (.. (class **WHAT-GOES-HERE**) getProtectionDomain getCodeSource getLocation) "!/META-INF/MANIFEST.MF")))) getMainAttributes) "Build-number")) Thanks.

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  • When should clojure keywords be in namespaces?

    - by Rob
    In clojure, keywords evaluate to themselves, e.g.: >>:test :test They don't take any parameters, and they aren't bound to anything. Why then, would we need to qualify keywords in a namespace? I know that creating isa hierachies using derive requires namespace qualified keywords. Are there any other cases where there is a clear need for keywords to be in a namespace?

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  • Project Euler #9 (Pythagorean triplets) in Clojure

    - by dbyrne
    My answer to this problem feels too much like these solutions in C. Does anyone have any advice to make this more lispy? (use 'clojure.test) (:import 'java.lang.Math) (with-test (defn find-triplet-product ([target] (find-triplet-product 1 1 target)) ([a b target] (let [c (Math/sqrt (+ (* a a) (* b b)))] (let [sum (+ a b c)] (cond (> a target) "ERROR" (= sum target) (reduce * (list a b (int c))) (> sum target) (recur (inc a) 1 target) (< sum target) (recur a (inc b) target)))))) (is (= (find-triplet-product 1000) 31875000)))

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  • Strange behaviour of keywords within macros in Clojure

    - by mikera
    I'm a little confused by how keyword accesses seem to behave in Clojure when they are evaluated at macro expansion time. The following works as I expect: (def m {:a 1}) (:a m) => 1 However the same keyword access doesn't seem to work within a macro: (def m {:a 1}) (defmacro get-a [x] (:a x)) (get-a m) => nil Any idea what is going on here?

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  • Type hinting for functions in Clojure

    - by mikera
    I'm trying to resolve a reflection warning in Clojure that seems to result from the lack of type inference on function return values that are normal Java objects. Trivial example code that demonstrates the issue: (set! *warn-on-reflection* true) (defn foo [#^Integer x] (+ 3 x)) (.equals (foo 2) (foo 2)) => Reflection warning, NO_SOURCE_PATH:10 - call to equals can't be resolved. true What is the best way to solve this? Can this be done with type hints?

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