Sunday, November 17, 2013

Why F# Type Provider?

F# type provider is a very nice feature. I was asked several times why I need type provider? Just because of the intellisense? I believe there should be something else. In the current team, I found people "hate" type, they prefer to use C# object all the times. Now I realize why type is so important and why type can increase code quality.

I want to use a simple approval process as a sample. For my understanding, a type defines the meaning of a area of memory and certain operations which can be executed on this type.

The sample is simple. You can have something to be approved by lead, manager, and director. If a lead approves, it returns a manager type. If manager disapproves, the process will stop and director won't know it. After director's approval, this process stops. The type provider can have three types, director's approve method returns nothing while other type's approve method return higher level type. Lead's approve method returns manager, and manager's approve method returns director.

I'd bet people can say I can use a single type to do the same job. A general class can be used to create three instances. The director instance's approve method can return NULL or any special value to stop the approval process. It can solve this problem provides you know director's approval will stop this process beforehand. A runtime error is not far away if this information is not known beforehand. The type provider uses type to provide the information. After you invoke director's approve method, which returns NULL, user will know immediately that this process is terminated.

I would say in this case, type is a way to move runtime check/error to compile time. The attached is the demo project for this approval process.

Friday, November 8, 2013

Use F# to Write PowerShell Snapin & Cmdlet

When I was in Minneapolis, I was thinking to use the F# to write PowerShell snapin and cmdlet. I got the feeling that this can speed our development speed.

Because I use Visual Studio 2012 and it generates the .NET 4 binary. So I need to install PowerShell 3.0 in order to use .NET 4.0. You can use $psversiontable to check the CLR runtime version, make sure it is a number equal or greater than 4.

 namespace Log4NetPsSnapIn  
 open System  
 open System.Management.Automation  
 open System.ComponentModel  
 [<RunInstaller(true)>]  
 type Log4NetSnapIn() =  
   inherit PSSnapIn()  
   override this.Name with get() = "aa"  
   override this.Vendor with get() = "bb"  
   override this.Description with get() = "dd"  
 [<Cmdlet(VerbsCommunications.Write, "Hi")>]  
 type WriteHelp() =   
   inherit Cmdlet()  
   override this.ProcessRecord() =   
     base.WriteObject("help");  

After compiling the above code, a DLL is generated. You have to use installutil.exe to add the snapin. The PowerShell snapin has Name = "aa", so you can use Add-PsSnapIn aa to load the DLL. You can also use installutil.exe /u to uninstall the snapin from your system. Make sure you open the cmd window or powershell window with administrator privilege.

  • If you are running 64-bit version machine, make sure you use installUtil.exe under C:\Windows\Microsoft.net\Framework64\. 
  • if your OS is 32-bit, you can use installUtil.exe under C:\Windows\Microsoft.net\Framework\


I like the way F# write PowerShell snapin. The code is concise and easier to understand.