Dixin's Blog

C#, .NET Core, Azure, Functional Programming, Lambda Calculus, Category Theory, LINQ, LINQ to Objects, LINQ to XML, Parallel LINQ, LINQ to Entities, Entity Framework Core, Azure SQL Database.

LINQ operators to enable C# 8.0 index and range new features working with LINQ queries and any type that implements IEnumerable<T>.

Microsoft has a C# example of uploading file to FTP server in MSDN https://msdn.microsoft.com/en-us/library/ms229715(v=vs.100).aspx and Microsoft docs https://docs.microsoft.com/en-us/dotnet/framework/network-programming/how-to-upload-files-with-ftp:

This article demonstrates how to setup a passive FTP server in a Azure virtual machine running Windows.

I use Windows’ Hyper-V to run virtual machines for long time. Recently I need to run a VMware virtual machine to test something. I installed VMware Player, which is free for non-commercial usage. However, the virtual machine cannot started, with an error:

Windows PE (WinPE) is a small version of Windows, which can be used to boot up computers from CD or USB disk drive. It is very useful to deploy or repair the desktop or server edition of Windows. For many years I have tried many options to build a WinPE image and create a bootable media, including the Microsoft official approach. The easiest way I found, is to use a third party tool called “Win10PE SE project”. I am sharing this tool with a post, because it is not easy to be found when searching WinPE related keywords.

In functional programming, there are many powerful tools and patterns, like lambda expression, purity, deferred execution, immutability, fluent LINQ query composition, … But everything has a cost. As Alan Perlis said:

In C#, callback is frequently used. For example, a very simple Add function, without asynchrony:

Unlike the Reader< , > monad, the Writer< , > monad output contents with a sequence of functions:

Sometimes there are functions work with a shared environment. Typical examples are:

State machine (or finite state machine) represents a abstract machine with one state or a number of state.  C# use state machine a lot. For example:

As mentioned in a previous part, in purely functional programming, functions cannot have side effects. For example, when defining LINQ queries, laziness and purity are expected. So, how should the impure actions be managed in purely functional programming or LINQ? For example:

Theoretically, Tuple<> should be counted as the Id<> monad. However, it is lack of laziness. In the context of C# and LINQ, it is only monad-like.

Again, Lazy<> is the simplest monad, it is just the lazy version of Tuple<>, and should be considered as the Id<> monad. This is the implementation of its SelectMany:

Previous part introduced SelectMany for monad IEnumerable<>. Actually SelectMany is more than meets the eye, and can be used to implement other LINQ queries.

A previous part showed endofunctor category is a monoid (the entire category itself). An endofunctor In the endofunctor category can be monoid too. This kind of endofunctor is called monad. Formally, monad is an endofunctor of category C, equipped with 2 natural transformations:

Theoretically, Tuple<> should be counted as the Id<> monoidal functor. However, as previously mentioned, it is lack of laziness.

Lazy<> should be the simplest monoid functor - it is just the lazy version of Tuple<>. And in these posts it will be considered as the Id<> monoidal functor.