f# essential - part 3 matching shortcut

F# provide a shortcut to define a function which purely use input parameter for matching purpose. For example the following are equal.

let printInt =
    function 1 -> printfn "1"
            | 2 -> printfn "2"
            | _ -> printfn "something else"

f# essential - part 2 - pipleline operator

Before I discuss pipeline operator, let's talk about operator. F# allow us to define operator as function. The following are infix operators

$ % & * + - . / < = > @ ^ | 

The following are prefix operators.

! ? ~

You can define a infix operator like below

let (operator) ops1 ops2 = ...
//for example 
let (+) a b = a - b
2+3

You can define a prefix operator like below

let (!) a = 1
!8

So you can change your expression from "add 2 4" to "2 + 3" or "m1 a" to "!a", let's defined our pipleine operator as below.

let (-->) x f = f x
let rb = 0.5 --> System.Math.Cos

However, F# already define the "|>" for us, so we should use "|>". Operator is not a function, definition, rather you should think of it as macro. The compiler will change the syntax before it compile.

f# essential - part 1 binding

I was very puzzled with some concept when learning F#. Here is some. Even I explain it here, you may be still confused, but it is ok, as you read on.

let t1 = 1
//fsi shows val t1 : int = 1
//think of it as (val t1) bind to (int = 1)

Based on the concept of other language, you think t1 is variable, and it is initialized or assigned with value 1. But here it should be understood as identifier t1 is bind to a value, the value is not function value, it is int 1. So what is the big deal of "identifier", "bind", "function value", and "value", we will see later. If you run the following expression in fsi

t1
//fsi shows val it: int =1

This is evaluation of expression "t1", the process is to bind value int 1 to unknown identifier. Let's read another example.

let t2() = 1
//fsi shows  val t2 : unit -> int
//here fsi should shows
//val t2: unit -> int = <fun:it@91-2>

Here it means identifier t2 is bind to a value, the value is a function value, and the value is a function take unit type parameter and return a int type value. Here unit is similar the concept of "void" in other language. How do I prove my remark "here fsi should shows...", because we can find out what is the value of t2, by evaluating expression "t2" like below, it return the value of t2, here what I really means "the value that t2 is bind to "

t2
//val it : (unit -> int) = <fun:it@91-2>

t2 is different from t1 in that the value t2 bind to is a function, while the value that t1 bind to is not. Because the value is function, we can call the function like below. The concept of "function is a value" is an important concept in functional language, it may not looks a big deal for you. But it is very powerful. Please note the function here is not a traditional function in VB, a method in c#, conceptually it is like C#'s delegate, or javascript's function, people call it lamda in other functional language.

t2();
//fsi shows val it : int = 1

The type description generated by fsi need some explanation.

t2();
int //int type
int -> string //function type take one int input, return one string
int -> string -> double // a function take one int, one string, return one double
int * string -> double // a function take a (int, string) tuple , and return one double

Semantics of undefined and null in javascript

Recently, I was discussing with a friend about the difference between undefined and null in javascript. I am surprised that I gave an explanation that he can understand, more surprisingly, I can understand too. Sometimes, people give an explanation, which they don't understand themselves, and also confuse others. Before going further, here is the explanation from JavaScript: The Definitive Guide

You might consider undefined to represent a system-level, unexpected, or error-like absence of value and null to represent program-level, normal, or expected absence of value. If you need to assign one of these values to a variable or property or pass one of these values to a function, null is almost always the right choice.

The explanation is confusing to me. What does this means in my daily coding, in what scenario I must use one but not the other or a scenario I can use either of them? So if null is almost the right choice, but why we have "undefined", is there any technical reason or semantic reason? Let's see why we have "undefined" value in javascript from technical perspective. When we write the following code, I can say, I declare two variables and they are not assigned with any value, and the default value of each is undefined.

var x;
var loooooooooooooooooooooooooong;

After reading the article Understand delete, I understand that, javascript variable mechanism. When you declared variable like above, you add a key/value pair entry to a mysterious object, VariableObject, which is dictionary. Here is pseudo code, that the engine will convert to

VariableObject["x"] = undefined;
VariableObject["loooooooooooooooooooooooooong"] = undefined;
//semantically, it means the following
//VariableObject.add("x", undefined);
//VariableObject.add("loooooooooooooooooooooooooong", undefined);

If the above example, two key/value pairs are added to the VariableObject dictionary. So both the key and value consume memory, so if you have a longer variable name, regardless its value, it use more memory than short variable. This is different from c++. In c++, a variable name is nick name of memory address. So practically, we should use shorter variable name, or use minifier to rename your variable. The VariableObject is special in that it is created by runtime. If the code is run in Global scope, the VariableObject is accessible as window. If it is run function scope, it is not accessible at all, which is known as Activation Object. Supposed it is run in global scope, it is same as the following.

window["x"] = undefined;
window["loooooooooooooooooooooooooong"] = undefined;

In the above case, variable x is said declared because its key is in the dictiobary, but its value is undefined. If the key is not even in the dictionary, then it is undeclared. Technically, there is difference between "undeclared" and "declared but undefined". But the following undefined check does not tell the difference.

//suggested by jQuery Code Style Guildeline
//http://docs.jquery.com/JQuery_Core_Style_Guidelines
//undefined check
//Global Variables: 
typeof variable === "undefined"
//Local Variables: 
variable === undefined
//Properties: 
object.prop === undefined

If you really need to know the difference, you need to use catch, because accessing undeclared variable directly will throw an exception.

function test(variableName) {
  try {
     var temp = eval(variableName);
    if (temp === undefined) {
       return "\"" + variableName + "\" is declared, its value is undefined"; 
    } else {
       return "\"" + variableName + "\" is declared, its value is not undefined"; 
    }
  } catch (e) {
      return "\"" + variableName + "\" is undeclared";
  }
}

matrix.js introduction

Essentially, matrix.js is like other script loader, but it does other things such extensible resource type, dependencies registration and reload, resource locations, resource release and others. In the following, I will explain what the library can do. All these example presented here can be found at GitHub