As I promised in my previous post, this post is dedicated to fully cover a practical application of DSL by combining 2 of my earlier posts.
http://joymonscode.blogspot.in/2013/01/what-is-domain-specific-language-dsl-in.html
http://joymonscode.blogspot.in/2012/12/running-powershell-script-from-c-vbnet.html
The idea was to give a kick start sample for C# developers into the world of DSL. But before going into the details I would like to add couple of theory titles.
How to select the DSL (Domain Specific Language)
This is a big question before you select the implementation method of DSL. According to Martin Fowler there are 2 categories of DSL. External and Internal DSL.
Internal DSL means adjusting our existing programming language practice to look more like business friendly. ie using plain English constructs to express the business cases. Linq is a best example for that. Also people are using fluent api for achieving internal DSLs. But here there is a considerable limitation. It is little difficult to add support for runtime changes. You can create the framework as a DSL and the tracks ie other developers can use that to achieve their tasks.
External DSL employees same programming language or different language altogether along with its own execution mechanism to run DSL program. SQL is a good example for that. We write C# code and for communicating with DB we need to write entirely different program called SQL queries. The execution engine and the model of SQL execution itself different from C#.
According to me if you are planning to select a DSL to conquer the constant requirements change better go for external DSL. This allows the end users, ability to change the program or behavior at run time. There is another psychological factor. If we ask your users write in C# or any other main stream programming language, they will protest first. But if you ask them to write in javascript or PowerShell they will be OK.This is because according to the end users, the responsibility of writing main stream code always lies with programmers. Can't believe?
This is proved last week in our office.We have an installation system which expects so many parameters to configure the application such as DB name, IIS application pool name etc...Earlier this has to be entered by the installation guy (QA people in case of testing) at the beginning of an .vbs file.That same vb script file contains the processing logic after the initial declaration section. They usually complain about this process of giving values in .vbs file. According to them its complicated. Now the process changed little bit. We leveraged MS Excel to enter parameter values and wrote vbs to read the values from there. They became very happy. The complication is washed out by introduction of excel. But did we do anything? Yes we removed a mental roadblock of modifying a program file by QA and other installation people. Still they need to enter DB name, application pool name etc...But at a different place.Now they are happy with the installation procedure.
Another thing to remember is, never go for creating your own parser or lexers to execute that external DSL. Of course if you have research time and lavish time you can go.But as an Architect / Technologist the best way is to leverage an existing well proven mechanism to achieve execution of DSL.
In the book "Wings of fire", former Indian president Dr. A.P.J Abdul Kalam is clearly defining the 'difference between science and technology'. According to him, if we are doing experiments to create something really new and we are not sure whether we can produce positive result, it is called as science. In the other hand, if we are doing something with limited resource and we must produce the positive output in time, its called technology.A scientist can go up to any limit to experiment what he wants. But technologist mostly works in constraints, so he needs to be aware of all the available proven options and use those wisely to achieve the goal.
Creating a new DSL and writing its parser and related things is a science. But leveraging the power of PowerShell, Javascript or any other language which can be embedded in our program to achieve the DSL execution is technology. Now you might have understood why the heading says PowerShell as DSL.
C# Salary calculation using PowerShell as DSL
In the previous post I had told about a salary calculation scenario where we can apply DSL to solve the requirement change issue. Basically requirement change is the nature of software engineering rather than issue. I am not good in explaining theories so I managed to create a C# program which uses PowerShell to accomplish the salary calculation based on dynamic logic written in PowerShell.
Why PowerShell
There is no special interest on PowerShell. You can select any scripting language such as Javascript, Lua etc..Only constraint is the scripting language should support embedding into C# application or there should be some methods to execute the script from C# and communicate back and forth.
The reasons for selecting PowerShell are
- Its becoming more and more popular like .bat file
- Easy to write programs using the pipline support.
- Recently I took a PowerShell hands on training session to a selected audience in our company :)
About the sample
Few words about the embedded sample. Its a simple program to demonstrate how to visualize the working of DSL. This is because most of the C# / VB.Net programmers might have heard about DSL but didn't experienced. It uses simple PowerShell invocation method from C#. In the formula textbox you can enter any PowerShell script snippet which returns a double value. The values for Basic, HRA & DA are available to the PowerShell context as variables $basic, $hra & $da respectively. Try changing the PowerShell code to calculate total salary by considering a 5% basic as metro allowance.
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