Function programming in C,

Function programming in C,

 

In function programming, functions can be considered as data. A function can also be used as a parameter. A function can also return a function. For example, LINQ is based on functional programming.

Two examples lead to functional programming

Statement-based programming may be written like this:

String result; if (value> 0) {result = "positive";} else {result = "negative ";}

 

Function expressions can be simplified:

Var result = value> 0 "positive": "negative ";

 

Let's look at an example of filtering and sorting:

var i = 0;while(i < list.Count){    if(list[i] % 2 != 0)    {        list.RemoveAt(i);    }    else    {        ++i;    }}list.Sort();

 

Function programming can be written as follows:

from x in list    where x  % 2 == 0    orderby x    select x;

 

Or

list    .where(x => x % 2 == 0)    .OrderBy(x => x)

 

It can be seen that in LINQ, the returned results of an expression (function) can be used as the source of an expression (function) and can be chained.

 

Encapsulate a Functional Method

For example, read remote data.

void Main(){    XDocument timeDoc;        using(var client = new System.Net.WebClient())    {        timeDoc = XDocument.Parse(client.DonwloadString(""));    }        var ms = Convert.ToInt64(timeDoc.Root.Attribute("time").Value) / 1000;    var currentTime = new DateTime(1977,1,1).AddMilliseconds(ms).ToLocalTime();    Console.WriteLine(currentTime);}

 

For the using part, we can extract a method.

private XDocument GetTime(){        using(var client = new System.Net.WebClient())    {        return XDocument.Parse(client.DonwloadString(""));    }    }void Main(){    var timeDoc = GetTime();    var ms = Convert.ToInt64(timeDoc.Root.Attribute("time").Value) / 1000;    var currentTime = new DateTime(1977,1,1).AddMilliseconds(ms).ToLocalTime();    Console.WriteLine(currentTime);   }

 

However, it is not enough. The above GetTime method only encapsulates the using statement that implements the IDisposable interface of WebClient. Can I encapsulate all the using statements that implement the IDisposable interface?

 

public static class MyDisposable{    public static TResult Using<TDisposable, TResult>(    Func<TDisposable> factory,     Func<TDisposable, TResult> map)     where TDisposable : IDisposable    {        using(var disposable = factory())        {            return map(disposable);        }    }}void Main(){    var time = MyDisposable        .Using(            () => new System.Net.WebClient(),            client => XDocument.Parse(client.DownloadString(""))        )        .Root        .Attribute("time")        .Value;            var ms = Convert.ToInt64(time) / 1000;    var currentTime = new DateTime().AddMilliseconds(ms).ToLocalTime();    Console.WriteLine(currentTime);}

 

In function programming, the type returned by a function is basically the source instance of another function method.