Clojure session #01

Since I’m not posting nothing here lately, I’ve decided to write some thoughts and examples with some subject, this time is Clojure.

Multiple arities

(defn sum
"sum given numbers"
([] 0)
([x] (+ x))
([x y] (+ x y)))

Undefined number of arguments

(defn sum [& all] (reduce + all))

Local bindings

This can help you to bind some values and let your code more readable

(let [x 2] (+ x 2)) ; x only exists within let context

doing sides effects (maybe you can see this like blocks)

(do
(println "hey")
(def a (read))
(println " you typed " a))

Closures in clojure

(defn validnumber? [max] #(if (> % max) false true))
; or (defn validnumber? [max] (fn [number] (if (> number max) false true)))

(def bellow100? (validnumber? 100))

(bellow100? 9) ; true
(bellow100? 200) ; false

Curying

(def plus-one (partial + 1))
(plus-one 2)

Compose functions

(def composed (comp #(+ 2 %) +))
(composed 1 2)

Thrush operators ( -> and ->> )

-> data flow (in front) among functions
->> data flow (after) among functions

(-> 4 (- 2) (- 2))
;translates to
(- 4 2) (- 2) => (- 2 2) => 0

(->> 4 (- 2) (- 2))
;translates to
(- 2 4) (- 2) => (- 2 -2) => 4

another good usage for ->

(def person {:name "name" :age 120 :location { :country "US" } })
;instead of (:country (:location person)) a more readable way could be
(-> person :location :country)

PS: hardly based on good book Pratical Clojure

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Will we only create and use dynamic languages in the future?

Since I’ve playing with some dynamic languages (Ruby and Clojure), I have been thinking about why would anybody create a new static typed language?! And I didn’t get the answer.

I started programming in Visual Basic and I taste its roots, which are almost all full of procedure commands (bunch of do, goto and end), then I moved to C#, sharper it changes the end’s for }’s and give us a little more power based on some premises: we can treat two different things in the same way, polymorphism. The last static language, but not the least, I used (and I use it) Java, abusing of his new way of treating a set of things equality, the interfaces and using its “powers” on reflections.

Although when I started to use Ruby I saw that I could treat a group of things equality without doing any extra work. I still need to code models and composed types, even though we can create or change them dynamically using “real power” of metaprogramming.

When I start to study and apply the Clojure and its principles, my first reaction was the rejection, how can I go on without my formal objects, how can I design software without a model in the head and so on. I wasn’t thinking about how actually I do software, currently I use TDD to design software and I don’t think what models I need to have, I do think in terms of “what I want”. At minimum, Clojure make me think about, do we really need object to design software?! .  A three days ago I saw an amazing video about similar thoughts: Some thoughts on Ruby after 18 months of Clojure.

Summarising: With my limited knowledge of theses languages, let’s suppose we use a function (which we don’t have source code) and we want to do something before that function is executed (intercept) using: VB I’ll need to check every single piece of code which we call this function and call another one, in Java we can use a AOP framework, in Ruby we can use the spells of metaprogramming. It seems that some frameworks, patterns and extra work aren’t needed more because of this dynamic language evolution.

My conclusions using dynamic languages (Clojure/Ruby) for now it’s: I write less code and reuse them more easy, so I don’t see any reason to create/use a new static typed language, would you see any motivation to do that?

PS: When I use C# (.Net Framework 1.3 – 2.0) it was not so super cool as today.

Clojure resources

Always that I start to learn a new language, I promise to keep the best resources links I found, but it never works. This post suppose to be updated often. Any broken link or suggestion, just comment and I’ll try to fix, add or remove it.

Links, tutorials, guides, documentations, screencasts and etc.

  1. Clojure official site
  2. Installing Clojure, clojure-contrib and setup EMACS.
  3. Vim and “Slime”
  4. VIM for Clojure
  5. Quick-start with examples
  6. VIDEO – Great short introduction videos for Clojure!
  7. VIDEO – Introduction to logic programming with Clojure
  8. VIDEO – Clojure for Java Programmers 1 of 2
  9. VIDEO – Functional programming by UCBerkeley 
  10. VIDEO – Great tutorial for Clojure focused on concurrency
  11. Try code clojure online
  12. Leiningen tutorial for beginners
  13. Midje – A test framework for Clojure
  14. Clojars – Community repository for open source clojure libraries

Books

Functional programming with Clojure

Clojure

I’ve been studying the new language called Clojure (all the cool kids are talking about Clojure). It is a functional language created by Rich Hickey around 2007. This is a(nother) dialect of Lisp. It is a dynamic language as Ruby, JavaScript and others. As said before Clojure (pronounced as closure) it’s a impure functional language in contrast with Haskell, a pure functional language. It runs over the JVM, so it’s fast, interoperable with Java among a lots of good stuffs that JVM give us. To put hands-on and try code something you can use the try Clojure online or you can download the clojure.jar file and run it. Surprisingly Clojure it’s easy to learn.


java -jar clojure-x.x.x.jar

What it a functional language? (concepts)

first-order functions -> functions are treated as values. You can store a function on a variable, you can pass one function to another or you can return a function from another function.

var sum = function(a,b){
  return a + b;
};

var obj = function(sum){
  return {
    hello: "hello",
    sum: sum
  };
}();

obj.sum(3,5);

functions constructs -> the language constructs are function instead of keyword. Constructions for conditions (if), for iterations (for, while), catch exceptions (try, catch) and others.


(if condition do-it else-do-it)

stateless -> it’s functional in the sense of math, you have functions which defines values input and output and doesn’t rely on outside global state. In such pure function you won’t produce any side-effect (read, write outside resource). Obviously we will produce programs which causes side-effects, clojure helps you build “mutable” data . On other pure languages like Haskell side-effects are treated as expections so you have concepts like actors and monad.

immutable data -> collections and local variable, in clojure, are immutable. The immutability, helps us in parallelism, since the “values” are immutable you can shared then without worry about locks.

currying -> is the technique of transforming a function that takes multiple arguments (or an n-tuple of arguments) in such a way that it can be called as a chain of functions each with a single argument (partial application).

memoization -> is an optimization technique used primarily to speed up computer programs by having function calls avoid repeating the calculation of results for previously processed inputs.

Resources