Merge pull request #1134 from NickPapanastasiou/master

D language!

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+---
+language: D 
+filename: learnd.d 
+contributors:
+    - ["Nick Papanastasiou", "www.nickpapanastasiou.github.io"]
+lang: en
+---
+
+```d
+// You know what's coming...
+module hello;
+
+import std.stdio;
+
+// args is optional
+void main(string[] args) {
+    writeln("Hello, World!");
+}
+```
+
+If you're like me and spend way too much time on the internet, odds are you've heard 
+about [D](http://dlang.org/). The D programming language is a modern, general-purpose,
+multi-paradigm language with support for everything from low-level features to 
+expressive high-level abstractions.
+
+D is actively developed by Walter Bright and Andrei Alexandrescu, two super smart, really cool
+dudes. With all that out of the way, let's look at some examples!
+
+```d
+import std.stdio;
+
+void main() {
+
+    // Conditionals and loops work as expected.
+    for(int i = 0; i < 10000; i++) {
+        writeln(i);
+    }
+
+    auto n = 1; // use auto for type inferred variables
+    
+    // Numeric literals can use _ as a digit seperator for clarity
+    while(n < 10_000) {
+        n += n;
+    }
+
+    do {
+        n -= (n / 2);
+    } while(n > 0);
+
+    // For and while are nice, but in D-land we prefer foreach
+    // The .. creates a continuous range, excluding the end
+    foreach(i; 1..1_000_000) { 
+        if(n % 2 == 0)
+            writeln(i);
+    }
+
+    foreach_reverse(i; 1..int.max) {
+        if(n % 2 == 1) {
+            writeln(i);
+        } else {
+            writeln("No!");
+        }
+    }
+}
+```
+
+We can define new types with `struct`, `class`, `union`, and `enum`. Structs and unions
+are passed to functions by value (i.e. copied) and classes are passed by reference. Futhermore,
+we can use templates to parameterize all of these on both types and values!
+
+```d
+// Here, T is a type parameter. Think <T> from C++/C#/Java
+struct LinkedList(T) {
+    T data = null;
+    LinkedList!(T)* next; // The ! is used to instaniate a parameterized type. Again, think <T> 
+}
+
+class BinTree(T) {
+    T data = null;
+    
+    // If there is only one template parameter, we can omit parens
+    BinTree!T left;
+    BinTree!T right;
+}
+
+enum Day {
+    Sunday,
+    Monday,
+    Tuesday,
+    Wednesday,
+    Thursday,
+    Friday,
+    Saturday,
+}
+
+// Use alias to create abbreviations for types
+
+alias IntList = LinkedList!int;
+alias NumTree = BinTree!double;
+
+// We can create function templates as well!
+
+T max(T)(T a, T b) {
+    if(a < b) 
+        return b;
+
+    return a;
+}
+
+// Use the ref keyword to ensure pass by referece.
+// That is, even if a and b are value types, they
+// will always be passed by reference to swap
+void swap(T)(ref T a, ref T b) {
+    auto temp = a;
+
+    a = b;
+    b = temp; 
+}
+
+// With templates, we can also parameterize on values, not just types
+class Matrix(uint m, uint n, T = int) {
+    T[m] rows;
+    T[n] columns;
+}
+
+auto mat = new Matrix!(3, 3); // We've defaulted type T to int
+
+```
+
+Speaking of classes, let's talk about properties for a second. A property
+is roughly a function that may act like an lvalue, so we can
+have the syntax of POD structures (`structure.x = 7`) with the semantics of
+getter and setter methods (`object.setX(7)`)!
+
+```d
+// Consider a class parameterized on a types T, U
+
+class MyClass(T, U) {
+    T _data;
+    U _other;
+
+}
+
+// And "getter" and "setter" methods like so
+class MyClass(T, U) {
+    T _data;
+    U _other;
+    
+    // Constructors are always named `this`
+    this(T t, U u) {
+        data = t;
+        other = u;
+    }
+    
+    // getters
+    @property T data() {
+        return _data;
+    }
+
+    @property U other() {
+        return _other;
+    }
+
+    // setters    
+    @property void data(T t) {
+        _data = t;
+    }
+
+    @property void other(U u) {
+        _other = u;
+    }
+}
+// And we use them in this manner
+
+void main() {
+    auto mc = MyClass!(int, string);
+
+    mc.data = 7;
+    mc.other = "seven";
+    
+    writeln(mc.data);
+    writeln(mc.other);
+}
+```
+
+With properties, we can add any amount of logic to
+our getter and setter methods, and keep the clean syntax of
+accessing members directly!
+
+Other object-oriented goodies at our disposal
+include `interface`s, `abstract class`es,
+and `override`ing methods. D does inheritance just like Java:
+Extend one class, implement as many interfaces as you please.
+
+We've seen D's OOP facilities, but let's switch gears. D offers
+functional programming with first-class functions, `pure` 
+functions, and immutable data. In addition, all of your favorite
+functional algorithms (map, filter, reduce and friends) can be
+found in the wonderful `std.algorithm` module!
+
+```d
+import std.algorithm : map, filter, reduce;
+import std.range : iota; // builds an end-exclusive range
+
+void main() {
+    // We want to print the sum of a list of squares of even ints
+    // from 1 to 100. Easy!
+    
+    // Just pass lambda expressions as template parameters!
+    // You can pass any old function you like, but lambdas are convenient here.
+    auto num = iota(1, 101).filter!(x => x % 2 == 0)
+                           .map!(y => y ^^ 2)
+                           .reduce!((a, b) => a + b);
+
+    writeln(num);
+}
+```
+
+Notice how we got to build a nice Haskellian pipeline to compute num? 
+That's thanks to a D innovation know as Uniform Function Call Syntax.
+With UFCS, we can choose whether to write a function call as a method
+or free function call! Walter wrote a nice article on this
+[here.](http://www.drdobbs.com/cpp/uniform-function-call-syntax/232700394) 
+In short, you can call functions whose first parameter 
+is of some type A on any expression of type A as a method.
+
+I like parallelism. Anyone else like parallelism? Sure you do. Let's do some!
+
+```d
+import std.stdio;
+import std.parallelism : parallel;
+import std.math : sqrt;
+
+void main() {
+    // We want take the square root every number in our array,
+    // and take advantage of as many cores as we have available.
+    auto arr = new double[1_000_000];
+
+    // Use an index, and an array element by referece,
+    // and just call parallel on the array!
+    foreach(i, ref elem; parallel(arr)) {
+        ref = sqrt(i + 1.0);
+    }
+}
+
+
+```