Wiesław Kałkus: C# functional programming

WIESŁAW KAŁKUS
Holte Software
Poland
Functional Programming
Is it possible in C#?

Today’s Agenda
C# Functional Programming
Is It Worth An Effort? Maybe It Is
Controlling Complexity
Composition Function Composition
Programming Example
Copying Photo Files Implementation Variants
What Is Functional Programming?
Theoretical Perspective C# Programmer Perspective

What is Functional
Programming?

What Is Functional Programming?
Wikipedia
Functional Programming is a programming paradigm,
a style of building the structure and elements of
computer programs, that treats computations as the
evaluation of mathematical functions and avoids
changing-state and mutable data. It is a declarative
programming paradigm, which means programming is
done with expressions.

Programming Paradigms
• Focus on describing how a program operates
• Programs described in terms of statements that change a program
state
• Structural (procedures as main actors) and object-oriented (objects as
main actors) programming styles fall into imperative paradigm
Imperative
• Focus on describing what a program should accomplish without
prescribing how to do it in terms of sequences of actions to be taken
• Programs expressed in terms of computation logic without describing
its control flow
• Functional (mathematical function as main actor) programming style
falls into declarative paradigm
Declarative

Multi-paradigm programming languages
Type inference (C# & F#)
Anonymous functions (C# & F#)
Language Integrated Query (C#
& F#)
Currying (F#)
Pattern matching (F#)
Lazy evaluation (F#)
Tail recursion (F#)
Immutability (F#)
Control statements (C#)
Classes (C#)
.NET framework (C# & F#)
Do notation (F#)
Declarative
Imperative

Object-Oriented vs. Functional Programming Style
Characteristic Object-oriented (imperative) approach Functional (declarative) approach
Programmer focus
How to perform tasks (algorithms) and
how to track changes in state.
What information is desired and what
transformations are required.
State changes Important. Localized and tightly controlled.
Order of execution Important. Low importance.
Primary flow control
Loops, conditionals, and function
(method) calls.
Function calls, including recursion.
Primary manipulation unit Instances of structures or classes.
Functions as first-class objects and data
collections.

Programming Example – problem statement
Copy photo files from the source folder (include subfolders) to the
destination folder (create subfolders as necessary). While copying
rename each file using pattern <base-name><sequence-number>.
Sequence numbers should preserve the order of date taken for each
photo. Additionally allow for each source photo file to create multiple
copies of the file in the target folder (useful for extending time-lapse
photography sequences).

Programming Example – object-oriented approach
foreach (sourceFile in sourceFilesSortedByPhotoDateTaken)
{
try
{
CreateTargetFolderPathIfNecessary
fileCopyCount = 0
while (fileCopyCount < PhotoFileCopyMultiplicationFactor)
{
MakeNewTargetFileName
CopySourceFileToTheTargetFolderUnderTheNewName
fileCopyCount += 1
}
}
catch
{
}
}
Programmer focus on how to perform
tasks (algorithms) and how to track
changes in state.

foreach (sourceFile in sourceFilesSortedByPhotoDateTaken)
{
try
{
fileCopyCount = 0
while (fileCopyCount < PhotoFileCopyMultiplicationFactor)
{
fileCopyCount += 1
}
}
catch
{
}
}
var sourceFiles = .Visit (sourceFolder);
sourceFiles.Sort( .ByDateTaken);
foreach (var sourceFile in sourceFiles) {
try {
fileCopyCount = 0
while (fileCopyCount < PhotoFileCopyMultiplicationFactor) {
fileCopyCount += 1
}
}
catch{
}
}
public static class FolderVisitor {
public static List<FileInfo> Visit(string rootFolder) {
return DoVisitFolder(rootFolder);
}
private static List<FileInfo> DoVisitFolder(string folder) {
var files = new List<FileInfo>();
var thisFolder = new DirectoryInfo(folder);
files.Add(thisFolder.EnumerateFiles());
var subFolders = thisFolder.EnumerateFolders();
foreach (var folder in subFolders) {
files.Add(DoVisitFolder(subfolder.FullName));
}
return files;
}
}
public static class Photo {
public static int ByDateTaken (FileInfo f1, FileInfo f2) {
if (f1 == null && f2 == null) {
return 0;
} else if (f1 == null) {
return -1;
} else if (f2 == null) {
return 1;
}
return Photo.DateTaken(f1).CompareTo(Photo.DateTaken(f1));
}
private static DateTime DateTaken(FileInfo f) {
DateTime dateTaken = DateTime.Now;
try {
using (var reader = ExifReader (f.FullName)) {
reader.GetTagValue<DateTime>(
ExifTags.DateTimeDigitized,
our dateTaken);
}
}
catch (IOException) {}
return dateTaken;
}
}
Add files from the
current folder.
Recursively add files
from subfolders.
Open source Exif Library
– read EXIF tag from the
photo file.

try {
FolderHelper.MaterializeFolder(targetFolder, sourceFile.Directory.FullName);
fileCopyCount = 0
fileCopyCount += 1
}
}
catch{
}
}
public static class FolderHelper {
public static void CopyFile(string targetRoot,
FileInfo sourceFile, string fileNameBase, int sequenceNo) {
var newFileName = string.Format(„{0}{1}{2}”,
fileNameBase, sequenceNo,
Path.GetExtension(sourceFile.Name));
var realtivePath = sourceFile
.Directory.FullName
.SubString(targetRoot.Length);
var targetPath = Path.Combine(targetRoot, relativePath,
newFileName);
sourceFile.CopyTo(targetPath);
}
}
Make target file
name
Extract target
relative path.
Create target
absolute path.Copy source file to
the target folder.

try {
FolderHelper.MaterializeFolder(targetFolder, sourceFile.Directory.FullName);
fileCopyCount = 0
FolderHelper.CopyFile(targetFolder, sourceFile,
newFileNameBase, sequenceNo);
fileCopyCount += 1;
sequenceNo += 1;
}
}
catch{
}
}
Make a list of source files.Sort source files by the
photo date taken.
Ensure target folder.
Make required file copies.
Update copying process state.

Programming Example – functional approach
• Make the list of source files
• Sort the source file list by picture date taken
• Make the list of pairs - source file and target
folder path
• Make the list of tuples – boolean flag
indicating sucessful file copy, source file,
copy operation status (destination path or
error message)
Programmer focus on what
information is desired and what
transformations are required.

folder path
error message)
• Make the list of source filesvar sourceFiles = .Visit (sourceFolder);
type public FileVisitor() =
static member private folderFiles folderPath filter =
let emptyDirectory = List.Empty.AsEnumerable()
let dirInfo = new DirectoryInfo(folderPath)
try
dirInfo.EnumerateFiles(filter)
with
| :? ArgumentException -> emptyDirectory
| :? DirectoryNotFoundException -> emptyDirectory
| :? SecurityException -> emptyDirectory
| :? UnauthorizedAccessException -> emptyDirectory
static member private subFolders folderPath =
let dirInfo = new DirectoryInfo(folderPath)
let noSubFolders = List.Empty.AsEnumerable()
try
dirInfo.EnumerateDirectories()
with
| :? DirectoryNotFoundException -> noSubFolders
| :? SecurityException -> noSubFolders
| :? UnauthorizedAccessException -> noSubFolders
static member public Visit rootFolder fileNameFilter =
seq { yield! FileVisitor.folderFiles rootFolder fileNameFilter
for subFolder in FileVisitor.subFolders rootFolder do
yield! FileVisitor.Visit subFolder.FullName fileNameFilter }
A sequence is a logical series of elements all of one
type. Sequences are particularly useful when you have
a large, ordered collection of data but do not
necessarily expect to use all the elements. Individual
sequence elements are computed only as required, so
a sequence can provide better performance than a list
in situations in which not all the elements are used.
var sourceFiles = FolderVisitor.Visit (sourceFolder);IEnumerable<FileInfo>

folder path
error message)
var sortedFiles = FolderVisitor.Visit (sourceFolder)
.OrderBy(Photo.DateTaken);
IEnumerable<FileInfo>
public static class Photo {
public static DateTime DateTaken(FileInfo f) {
DateTime dateTaken = DateTime.Now;
try {
using (var reader = ExifReader (f.FullName)) {
reader.GetTagValue<DateTime>(
ExifTags.DateTimeDigitized,
our dateTaken);
}
}
catch (IOException) {}
return dateTaken;
}
}

• Make the list of pairs - source file and
target folder path
var sortedFiles = FolderVisitor.Visit (sourceFolder)
.OrderBy(Photo.DateTaken);
error message)
var copier =new (targetFolder, baseName, seqStart, noCopies);
var toBeCopiedFiles = FolderVisitor.Visit (sourceFolder)
.OrderBy(Photo.DateTaken)
.SelectMany(copier.MakeTargetPath);
public class FileCopier {
private readonly string _targetFolder;
private readonly string _baseName;
private readonly int _noOfCopies;
private int _sequenceNo;
public FileCopier(string targetFolder, string baseName, int seqStart, int noOfCopies) {
_targetFolder = targetFolder;
_baseName = baseName;
_noOfCopies = noOfCopies;
_sequenceNo = seqStart;
}
public IEnumerable<Tuple<FileInfo, string>> MakeTargetPath(FileInfo file) {
var result = new List<Tuple<FileInfo, string>();
for (var i = 0; i < _noOfCopies; ++i) {
var newFileName = string.Format(„{0}{1}{2}”, _baseName, _sequenceNo,
Path.GetExtension(file.Name));
var realtivePath = file.Directory.FullName.SubString(_targetFolder.Length);
var targetPath = Path.Combine(_targetFolder, relativePath, newFileName);
result.Add(new Tuple<FileInfo, string>(file, targetPath));
_sequenceNo += 1;
}
return result;
}
}
var copier =new FileCopier(targetFolder, baseName, seqStart, noCopies);
IEnumerable<Tuple<FileInfo, string>>
Projects each element of a sequence to an IEnumerable<T> and
flattens the resulting sequences into one sequence.
public static IEnumerable<TResult> SelectMany<TSource, TResult>(
this IEnumerable<TSource> source,
Func<TSource, IEnumerable<TResult>> selector
)

error message)
var fileCopyStatus = FolderVisitor.Visit (sourceFolder)
.SelectMany(copier.MakeTargetPath)
.SelectMany(copier.CopyFile);
public class FileCopier {
private readonly string _targetFolder;
private readonly string _baseName;
private readonly int _noOfCopies;
private int _sequenceNo;
public FileCopier(string targetFolder, string baseName, int seqStart, int noOfCopies) {
_targetFolder = targetFolder;
_baseName = baseName;
_noOfCopies = noOfCopies;
_sequenceNo = seqStart;
}
public IEnumerable<Tuple<bool, FileInfo, string>> CopyFile(Tuple<FileInfo, string> file) {
var result = new List<Tuple<bool, FileInfo, string>();
try {
MaterializeFolder(_targetFolder, file.Item2);
file.Item1.CopyTo(file.Item2);
result.Add(new Tuple<bool, FileInfo, string>(true, file.Item1, file.Item2);
} catch (Exception ex) {
result.Add(new Tuple<bool, FileInfo, string>(false, file.Item1, ex.Message);
}
return result;
}
}
IEnumerable<Tuple<bool, FileInfo, string>>

Programming Example – Object-Oriented vs Functional approach
var sourceFiles =
FolderVisitor.Visit (sourceFolder);
try {
FolderHelper.MaterializeFolder(targetFolder,
sourceFile.Directory.FullName);
fileCopyCount = 0
while (fileCopyCount < noOfCopies) {
FolderHelper.CopyFile(targetFolder,
sourceFile,
newFileNameBase,
sequenceNo);
fileCopyCount += 1;
sequenceNo += 1;
}
}
catch{
}
}
var copier = new FileCopier(targetFolder,
baseName,
seqStart,
noCopies);
var fileCopyStatus =
FolderVisitor.Visit (sourceFolder)
var fileCopyObserver = new FileCopyObserver();
Control
statements,
state changes Function call
composition
Controlling Complexity By

Composition
Composition is the key to controlling complexity in software.
Expert programmers control the complexity of their designs by
combining primitive elements to form compound objects, they
abstract compound objects to form higher-level building blocks.
One form of composition, function composition, describes the
dependencies between function calls.

Function Composition
Function composition takes two functions and passes the result of
the second function as the argument of the first function –
effectively creating one function taking argument of the second
function and returning result of the first function.
public static Func<T, V> Compose<T, U, V>(this Func<U, V> f, Func<T, U> g)
{
return x => f(g(x));
}
Anonymous function (lambda) calling second function (g) with argument x and
passing its result as first function (f) argument, and returning the result of the
first function.

Function Composition
var r = f(g(x))
•Explicit
dependency
between f and g
var r = f.Compose(g)(x)
•Abstracted
dependency
between f and g
Abstract compound objects to form higher-level building blocks

Function Composition Laws
• Identity.Compose(f) = f
Left
identity
• f.Compose(Identity) = f
Right
identity
• f.Compose(g.Compose(h) =
(f.Compose(g)).Compose(h)Associative
public static T Identity<T>(this T value)
{
return value;
}

Function Composition – sometimes values are not enough
Generic (constructed) types package (amplify) values (integers,
strings, objects of class T)
The type IEnumerable<T> represents a lazily computed list of values
of type T
The type IEnumerable<T> packages (amplifies) the type T

Function Composition with IEnumerable<T>
public static Func<T, IEnumerable<V>> Compose<T, U, V>(
this Func<U, IEnumerable<V>> f,
Func<T, IEnumerable> g)
{
return x => f(g(x));
}
Error – g(x) returns IEnumerable while f takes U
We need to bind the result of g(x) with the function f. Binding will
iterate over a list returned by g(x) and pass individual list items to
the function f. The result of binding g(x) to the function f is a list of
results returned by the function f.
public static Func<T, IEnumerable<V>> Compose<T, U, V>(
this Func<U, IEnumerable<V>> f,
Func<T, IEnumerable> g)
{
return x => g(x).Bind(f);
}
public static IEnumerable<V> Bind (
this IEnumerable m,
Func<U, IEnumerable<V>> f)
public static IEnumerable<V> Bind (this IEnumerable items, Func<U, IEnumerable<V>> f)
{
var result = new List<V>();
foreach (var item in items)
{
result.Add(f(item));
}
return result;
}

Function Composition with IEnumerable<T>
• Our Bind function allows the function f to use IEnumerable returned by the function g
• We need some function converting a type T to IEnumverable<T>
public static IEnumerable<T> Unit (this T value)
• Together the amplified type IEnumerable<T>,
the function Bind, and the function Unit enable
function composition with amplified values
A
type
A Unit
function
A Bind
function
Monad
Monads enable function
composition with amplified values

Monads Satisfy Function Composition Laws
•Unit(v).Bind(f) = f(v)Left identity
•m.Bind(Unit) = mRight identity
•m.Bind(x => k(x).Bind(y => h(y)) =
m.Bind(x =>k(x)).Bind(y => h(y))
Associative

Creating a Monad
To define a particular monad, the writer
supplies the triple (a Type, a Unit function,
and a Bind function), thereby specyfying
the mechanics of the aplified values

IEnumerable<T> Monad
IEnumerable<T> Monad
SelectMany
function
AsEnumerable
function
Type
T

C# Functional Programming
• Is it worth an effort?
• Maybe it is
• The Maybe monad is an example of a monadic
container type wrapping a value. The container
can either have a value or have missing value.
The Maybe monad is a better nullable type.

C# Functional Programming – Maybe monad
public class Maybe<T> {
public readonly static Maybe<T> Nothing = new Maybe<T>();
public bool IsNothing { get; private set; }
public T Just { get; private set; }
public Maybe() {
IsNothing = true;
Just = default(T);
}
public Maybe(T value) {
IsNothing = false;
Just = value;
}
public new string ToString() {
if (IsNothing) {
return "Nothing";
}
return Just.ToString();
}
}
Nothing represents all instances lacking a value.
Just the value if we have one.
We have the type. Now we need the Unit and
Bind functions to have the Maybe monad.
The Unit function which we call AsMaybe,
wraps a value.
The Bind function which we call SelectMany,
takes a Maybe instance and if there is a value
then it applies the delegate to the contained
value. Otherwise, it returns Nothing.

Maybe monad – Unit and Bind functions
public static class MaybeExtensions {
public static Maybe<T> AsMaybe<T> (this T value){
return new Maybe<T>(value);
}
public static Maybe SelectMany<T, U>(this Maybe<T> m,
Func<T, Maybe> doSemthingWith) {
if (m == null) {
return Maybe.Nothing;
}
if (m.IsNothing) {
}
try {
return doSomethingWith(m.Just);
}
catch (Exception) {
}
}
}
The Unit function which we call AsMaybe, wraps
a value.
The Bind function which we call SelectMany,
takes a Maybe instance and if there is a value
then it applies the delegate to the contained
value. Otherwise, it returns Nothing.
The delegate takes the un-wrapped value of the
type T and returns wrapped (amplified) value of
the type U – we can bind another delegate to
ther result Maybe.

Maybe monad – usage examples
var maybeStringArray = (new string[] {„Ala”, „ma”, „kota”}).AsMaybe();
var maybeResult = maybeStringArray
.SelectMany(x => x.FirstOrDefault(s => s.Equals(„Ala”)).AsMaybe())
.SelectMany(s => s.ToUpperInvariant().AsMaybe());
Console.WriteLine(maybeResult.ToString());
maybeResult = maybeStringArray
.SelectMany(x => x.FirstOrDefault(s => s.Equals(„Ola”)).AsMaybe())
.SelectMany(s => s.ToUpperInvariant().AsMaybe());
Console.WriteLine(maybeResult.ToString());
Writes ALA.
Writes Nothing.
Maybe<string>

Wiesław Kałkus: C# functional programming

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