Rash thoughts about .NET, C#, F# and Dynamics NAV.

Yesterday I was talking about F# at the .NET Developer Group Braunschweig. It was my first talk completely without PowerPoint (just Live-Coding and FlipChart) and I have to admit this is not that easy. But the event was really a big fun and we covered a lot of topics like FP fundamentals, concurrency and domain specific languages (of course I showed “FAKE – F# Make”).

Now I have a bit time before I go to the next BootCamp in Leipzig. Today Christian Weyer will show us exciting new stuff about WCF and Azure.

In the meanwhile I will write here about another important question (see first article) from the F# BootCamp in Leipzig:

Question 4 – Try to explain “Currying” and “Partial Application”. Hint: Please show a sample and use the pipe operator |>.

Obviously this was a tricky question for FP beginners. There are a lot of websites, which give a formal mathematical definition but don’t show the practical application.

“Currying … is the technique of transforming a function that takes multiple arguments (or more accurately an n-tuple as argument) in such a way that it can be called as a chain of functions each with a single argument”

[Wikipedia]

I want to show how my pragmatic view of the terms here, so let’s consider this small C# function:

public int Add(int x, int y)
{
   return x + y;
}

Of course the corresponding F# version looks nearly the same:

let add(x,y) = x + y

But let’s look at the signature: val add : int * int –> int. The F# compiler is telling us add wants a tuple of ints and returns an int. We could rewrite the function with one blank to understand this better:

let add (x,y) = x + y

As you can see the add function actually needs only one argument – a tuple:

let t = (3,4)         // val t : int * int
printfn "%d" (add t)  // prints 7 – like add(3,4)

Now we want to curry this function. If you’d ask a mathematician this a complex operation, but from a pragmatic view it couldn’t be easier. Just remove the brackets and the comma – that’s all:

let add x y = x + y

Now the signature looks different: val add : int -> int –> int

But what’s the meaning of this new arrow? Basically we can say if we give one int parameter to our add function we will get a function back that will take only one int parameter and returns an int.

let increment = add 1      // val increment : (int -> int)
printfn "%d" (increment 2) // prints 3

Here “increment” is a new function that uses partial application of the curryied add function. This means we are fixing one of the parameters of add to get a new function with one parameter less.

But why are doing something like this? Wouldn’t it be enough to use the following increment function?

let add(x,y) = x + y       // val add : int * int -> int
let increment x = add(x,1) // val increment : int -> int
printfn "%d" (increment 2) // prints 3

Of course we are getting (nearly) the same signature for increment. But the difference is that we can not use the forward pipe operator |> here. The pipe operator will help us to express things in the way we are thinking about it.

Let’s say we want to filter all even elements in a list, then calculate the sum and finally square this sum and print it to the console. The C# code would look like this:

var list = new List<int> {4,2,6,5,9,3,8,1,3,0};
Console.WriteLine(Square(CalculateSum(FilterEven(list))));

If we don’t want to store intermediate results we have to write our algorithm in reverse order and with heavily use of brackets. The function we want to apply last has to be written first. This is not the way we think about it.

With the help of curried functions, partial application and the pipe operator we can write the same thing in F#:

let list = [4; 2; 6; 5; 9; 3; 8; 1; 3; 0]

let square x = x * x

list
 |> List.filter (fun x -> x % 2 = 0) // partial application
 |> List.sum
 |> square
 |> printfn "%A"                     // partial application

We describe the data flow in exactly the same order we talked about it. Basically the pipe operator take the result of a function and puts it as the last parameter into the next function.

What should we learn from this sample?

1. Currying has nothing to do with spicy chicken.
2. The |> operator makes life easier and code better to understand.
3. If we want to use |> we need curryied functions.
4. Defining curryied functions is easy – just remove brackets and comma.
5. We don’t need the complete mathematical theory to use currying.
6. Be careful with the order of the parameter in a curryied function. Don’t forget the pipe operator puts the parameter from the right hand side into your function – all other parameters have to be fixed with partial application.

Easy TeamCity integration is one of the major goals for the FAKE build system.

“TeamCity is a Java-based build management and continuous integration server from JetBrains, creators of IntelliJ IDEA and ReSharper.”

[Wikipedia]

In this article I will show you how you can set up a FAKE build script in TeamCity. We will use the CalculatorSample which you can download from the FAKE Download page. If you want to know how this build script works read the “Getting started with FAKE”-tutorial.

Installing TeamCity

You can download the free professional edition of TeamCity from http://www.jetbrains.com/teamcity/. After the installation process (further instructions) you should be ready to configure your first build.

Creating a FAKE Project

Now create a new project and add a build configuration:

Attach a VCS root

The next step is to attach a VCS root. For this sample we will use the official FAKE repository at https://github.com/forki/FAKE/.

Choosing a build runner

We can use the Command Line build runner to start the completeBuild.fsx build script via Fake.exe.

If you want you could also add a build trigger to your build script:

Running the build

Now if everything is configured correctly, you can run your build and the output should look like:

You can also inspect the NUnit and FxCop results:

“FAKE – F# Make” is intended to be an extensible build framework. That’s why I tried to make it as easy as possible to create custom tasks. This posts shows how we can create a (very simple) custom task in C# which gives a random number.

Open Visual Studio and create a new C# class library called my MyCustomTask and create a class called RandomNumberTask:

Now compile this project and put the generated assembly into the tools/FAKE path of your project. Now you can use your CustomTask in the build script:

You can use every .Net class with FAKE. Just put the assembly in the right folder (tools/FAKE) and include it with the #r command.

If you want to use FAKE’s standard functionality (like globbing) within your CustomTask project, just reference FakeLib.dll and explore the FAKE namespace.

In the last article I showed how we can use “FAKE – F# Make” to set up a build script which

1. Cleans up old build outputs
2. Compiles our main projects
3. Compiles test projects
4. Uses NUnit to test our assembly
5. Zips the assemblies to a deploy folder.

This time we will improve the same Calculator sample (download here) with a task for FxCop, so please make sure you succeeded with the last article.

“FxCop is a free static code analysis tool from Microsoft that checks .NET managed code assemblies for conformance to Microsoft’s .NET Framework Design Guidelines.”

[Wikipedia]

Setting up FxCop

Open build.fsx from your Calculator sample folder and add a new target “FxCop” to the targets section:

In the dependencies section modify the build order to:

That’s it. If you run your build script you will get new *.xml file in the .\test\-folder:

There are a lot of parameters for the FxCop task. Some are described on the project page.

Letting the build fail

If you were using MSBuild before you might know how hard it is to let MSBuild fail your build if FxCop reports any errors or warnings.

With FAKE the only thing you have to do is setting the “FailOnError” parameter:

If you activate this option FxCop errors will cause your build to fail. Possible values are:

• FxCopErrorLevel.Warning
• FxCopErrorLevel.CriticalWarning
• FxCopErrorLevel.Error
• FxCopErrorLevel.CriticalError
• FxCopErrorLevel.ToolError
• FxCopErrorLevel.DontFailBuild

The values are cummulative. If you choose FxCopErrorLevel.CriticalWarning the build will fail for critical warnings, errors, critical errors and FxCop tool errors but not for simple warnings. The default is FxCopErrorLevel.DontFailBuild.

In this tutorial I will describe how you can set up a complete build infrastructure with “FAKE – F# Make”. You will learn:

• How to automatically compile your C# or F# projects
• How to automatically run NUnit UnitTests on your projects
• How to zip the output to a deployment folder

Install F#

“FAKE – F# Make” is completely written in F# and all build scripts will also be written in F#, but this doesn’t imply that you have to learn programming in F#. In fact the “FAKE – F# Make” syntax is hopefully very easy to learn. But if you need to you can use the complete power of F# and the .NET Framework.

But in order to get things working we need to install the F# environment. You can download the F# April 2011 CTP from the Microsoft F# Developer Center or install Visual Studio 2010.

Download Calculator Sample

Now download the latest CalculatorSample-*.zip from the FAKE Download site. This sample includes 3 tiny projects and basically the following structure:

• src\app
  • Calculator (Command line)
  • CalculatorLib (Class library)
• src\test
  • Test.CalculatorLib (NUnit test library)
  • xUnit.Test.CalculatorLib (xUnit test library)
• tools
  • FAKE Assemblies
  • NUnit
• build.bat
• build.fsx
• completeBuild.bat
• completeBuild.fsx
• Calculator.sln

Getting “FAKE – F# Make” started

If you run the build.bat from the command line then your first FAKE script (build.fsx) will be executed. If everything works fine you will get the following output:

Now open the build.fsx with Visual Studio. It should look like this:

As you can see the code is really simple. The first lines include the FAKE library and are vital in all FAKE build scripts.

After this header the Default target is defined. A target definition contains of two important parts. The first is the name of the target (here “Default”) and the second is an action (here a simple trace of “Hello world”). The Action can be defined as simple lambda expressions or as methods.

The last line runs the “Default” target – which means it executes the defined action of the target.

Cleaning the last build output

A typical first step in most every build scenario is to clean the output of the last build. We can achieve this by modifying the build.fsx to the following:

We introduced some new concepts in this snippet. At first we defined a global property called “buildDir” with the relative path of a temporary build folder.

In the Clean target we use the CleanDir task to clean up this build directory. This simply deletes all files in the folder and creates the directory if necessary.

In the dependencies section we say that the Default target is dependent of the Clean target. In other words Clean is a prerequisite of Default and will be run before the execution of Default:

Building the application

In the next step we want to compile our application libraries, which means we want to compile all projects under /src/app with MSBuild.

We defined a new build target named “BuildApp” which compiles all project files given in the property appReferences with the MSBuild task and the build output will be copied to buildDir.

In order to find the right project files “FAKE – F# Make” scans the folder src/app/ and all subfolders with the given pattern. Therefore a similar FileSet definition like in NAnt or MSBuild (see project page for details) is used.

In addition the target dependencies are modified again. Now Default is dependent of BuildApp and BuildApp needs Clean as a prerequisite.

This means the execution order is: Clean ==> BuildApp ==> Default.

Building Test projects

Now our main application will be built automatically and it’s time to build the test application. We use the same concepts as before:

This time we defined a new target “BuildTest”, which compiles all C# projects below src/test/ in Debug mode and we put the target into our build order.

Testing the test assemblies with NUnit

Now all our projects will be compiled and we can use the NUnit task in order to let NUnit test our assembly:

Our new target “Test” scans the test directory for test assemblies and runs them with NUnit. The mysterious part (fun p –> …) simply overrides the default parameters of the NUnit task and allows to specify concrete parameters..

Deploying a zip file

Now we want to deploy a *.zip file containing our application:

The new target “Deploy” scans the build directory for all files. The result will be zipped to \deploy\Calculator.zip via the Zip task.

What’s next?

Now you are ready to write your own “FAKE – F# Make” build scripts. If you have any questions or suggestions feel free to comment on this post.

In the next article I will show how we can add FxCop to our build in order to check specific naming rules.