Add Structures

Structures.md

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+Structures allow coders to define their own types. They are similar to lists, but they do have a few key differences:
+
+- Each field of a set can be called at any time
+- You can define functions within the structure, which can be called by using `!struct.fun`.
+- The fields of a structure are unordered
+
+Let's have a look at how you can create a structure.
+
+## The Basics
+For our example, we will be making a point structure. This will store a point on the Cartesian plane.
+
+Let's create the framework for the struct:
+```
+struct -point
+
+endstruct
+```
+All this does is create a new structure, named point.
+
+## Defining Fields
+A field is, to put it simply, the parts that make up a structure. To define a field, use `init &fieldname -type`.
+
+For our example, we want to store two variables that store a double: one for the x-position, and one for the y-position. So our fields will be xpos and ypos.
+
+```
+struct -point
+    init &xpos -double
+    init &ypos -double
+    
+endstruct
+```
+Now we have a complete structure!
+
+## Defining a variable as a structure
+We have a point, but how can we set a variable as a point? It is pretty simple: just use `init &myPoint -point`.
+
+To modify the xpos and ypos of the `myPoint` variable, you can use `set &myPoint.xpos $value` and `set &myPoint.ypos $value`.
+```
+struct -point
+    init &xpos -double
+    init &ypos -double
+    
+endstruct
+
+init &myPoint -point
+set &myPoint.xpos 3
+set &myPoint.ypos 4
+```
+This stores the point (3, 4).
+
+And now you know the basics of making a structure! You know how to define a structure, its fields, and how to define a variable as a structure. But there is more you can do...
+
+## Member Functions
+One trick you can do is to define functions within the structure. Typically these will be getting some value related to the structure.
+
+For our example, we will find the distance of the point from the origin. Let's first build the framework.
+
+```
+struct -point
+    init &xpos -double
+    init &ypos -double
+    
+    fun -double !modulus
+        
+    endfun
+endstruct
+```
+
+The distance from the origin, by the Pythagorean Theorem, is $$\sqrt{x^2+y^2}$$. We can simply use `$xpos` and `$ypos` to find the respective values when the function is called.
+
+We will import the math function to use the square root function.
+
+```
+struct -point
+    init &xpos -double
+    init &ypos -double
+    
+    fun -double !modulus
+        multiply $xpos $xpos &x2
+        multiply $ypos $ypos &y2
+        add $x2 $y2 &z2
+        pusharg $z2
+        !math:sqrt &z
+        return $z
+    endfun
+endstruct
+```
+
+Now, if we run `!myPoint.modulus`, we can get the modulus of the vector represented by `($myPoint.xpos, $myPoint.ypos)`.
+
+And that's all you need to know!
+
+## Other examples of structures
+
+- A user account for a messaging service
+```
+struct -user
+    init -string &username
+    init -string &password
+    init -bool &isBanned
+endfun
+```