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

I just released a new version of my Open Source Build Automation Framework “FAKE – F# Make”. You can read more about FAKE on the project website or in the Getting started with "FAKE – F# Make"-article.

Although the new release contains many bugfixes, I only want to show the two major improvements here.

1. FAKE 0.10 uses FSI instead of FSC

From now on FAKE uses the “F# Interactive” (fsi.exe) instead of the F# Compiler (fsc.exe) to run the build scripts, which brings two major improvements.

No TempPath for compiled binaries needed

Due to the fact that FAKE scripts are no longer compiled at the beginning of the build process, we don’t need a temporary folder for the created binaries.

Loading modules at runtime

The #load command in F# scripts allows us to load modules at runtime. Now we are able to put reusable Targets or TargetTemplates (see below) into external build script files.

2. TargetTemplates

TargetTemplates provide an easy way to reuse common Targets. Let’s consider a (very) small sample:

This Target “TraceHello” traces a “Hello World” string into our build log. Now we want it to be slightly more generic and to trace a custom string. We can do this by using a TargetTemplate:

Now we have a template (or a function which generates targets) that gets a string for the target name and a string for the trace text and generates  a usable target:

Of course the TargetTemplate function is generic and can be used with any tuple as parameter:

One of the nice properties of functional programming languages is the easy extensibility of custom functions. Let’s consider a simple F# function (from “FAKE – F# Make”) for a recursive directory copy:

open System
open System.IO

/// Copies a directory recursive
/// Thanks to Robert Pickering http://strangelights.com/blog/
///  param target: target directory : string
///  param source: source directory : string
let CopyDir target source =
  Directory.GetFiles(source, "*.*", SearchOption.AllDirectories)
    |> Seq.iter (fun file -> 
      let newFile = target + file.Remove(0, source.Length)
      printf "%s => %s" file newFile
      Directory.CreateDirectory(Path.GetDirectoryName(newFile)) |> ignore
      File.Copy(file, newFile, true))

If we want to allow users to set custom file filters, we can add a third parameter:

/// Copies a directory recursive
/// and allows to filter the files
/// Thanks to Robert Pickering http://strangelights.com/blog/
///  param target: target directory : string
///  param source: source directory : string
///  param filterFile: FilterFunction: string -> bool
let CopyDirFiltered target source filterFile =
  Directory.GetFiles(source, "*.*", SearchOption.AllDirectories)
    |> Seq.filter filterFile
    |> Seq.iter (fun file -> 
      let newFile = target + file.Remove(0, source.Length)
      printfn "%s => %s" file newFile
      Directory.CreateDirectory(Path.GetDirectoryName(newFile)) |> ignore
      File.Copy(file, newFile, true))

Now we can define some filter functions:

/// Exclude SVN files (path with .svn)
/// excludeSVNFiles: string -> bool 
let excludeSVNFiles (path:string) = not <| path.Contains ".svn"

/// Includes all files
/// allFiles: string -> bool 
let allFiles (path:string) = true

Now it is possible to use CopyDirFiltered in the following ways:

/// Copies all files <=> same as CopyDir
CopyDirFiltered "C:\\target" "C:\\source" allFiles

/// Copies all files except SVN files
CopyDirFiltered "C:\\target" "C:\\source" excludeSVNFiles

/// Copies all files only if random number <> 2
let r = new Random()
CopyDirFiltered "C:\\target" "C:\\source" (fun path -> r.Next(5) <> 2)

Extensibility of functions in C#

Of course we can do the same thing in C# 3.0:

/// <summary>
/// Copies a directory recursive
/// and allows to filter the files
/// </summary>
/// <param name="target">The target.</param>
/// <param name="source">The source.</param>
/// <param name="fileFilter">The file filter.</param>
public static void CopyDirFiltered(string target, string source,
                                   Func<string, bool> fileFilter)
{
    string[] allFiles = Directory.GetFiles(
        source, "*.*", SearchOption.AllDirectories);
    foreach (string file in from f in allFiles
                            where fileFilter(f)
                            select f)
    {
        string newFile = target + file.Remove(0, source.Length);
        Console.WriteLine("{0} => {1}", file, newFile);
        Directory.CreateDirectory(Path.GetDirectoryName(newFile));
        File.Copy(file, newFile, true);
    }
}

Now it is easy to use the C# function with lambdas:

“A lambda expression is an anonymous function that can contain expressions and statements, and can be used to create delegates or expression tree types.”

[MSDN]

Func<string, bool> filterSVN = x => !x.Contains(".svn");
Func<string, bool> allFiles = x => true;

/// Copies all files <=> same as CopyDir
CopyDirFiltered("C:\\target", "C:\\source", allFiles);

/// Copies all files except SVN files
CopyDirFiltered("C:\\target", "C:\\source", filterSVN);

/// Copies all files only if random number <> 2
var r = new Random();
CopyDirFiltered("C:\\target", "C:\\source", path => r.Next(5) != 2);

Keeping this simple technique in mind allows to create very flexible functions.

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.

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This article has been moved to http://fsharp.github.io/FAKE/gettingstarted.html