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.

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