Mixing functional and object oriented approaches to programming in C#

Mixing functional and object oriented approaches to programming in C# Mark Needham

A bit of context

ThoughtWorks delivery projects

Web applications with somewhat complicated domains

5-15 developers on a team

Projects running for 6-12 months

All this means that we want to write code which is…

Easy to understand

Easy to change

Which leads us to what this talk is all about…

Organisation of code
‘ The Lush Landscape of Languages’ - The ThoughtWorks Anthology
Rebecca Parsons

How might functional programming help us with that?

First class functions

“ A programming language is said to support first class functions if functions can be created during the execution of a program, stored in data structures, passed as arguments to other functions, and returned as the values of other functions”
Wikipedia

Immutability

Lazy evaluation

Recursion

Pattern matching

This is all very cool but…

Object Oriented design still has its place

Encapsulation

Abstraction

So how do these two paradigms work together?

Programming in the small/medium/large
http://weblogs.asp.net/podwysocki/archive/2009/12/14/going-hybrid-implementing-a-shopping-cart-in-f.aspx

In the large… “a high level that affects as well as crosscuts multiple classes and functions”

In the medium… “a single API or group of related APIs in such things as classes, interfaces, modules”

In the small… “individual function/method bodies”

Large Medium Small

a.k.a. Functional collection parameters
http://billsix.blogspot.com/2008/03/functional-collection-patterns-in-ruby.html

“powerful abstractions over collections”

“ the use of high-order functions is extremely useful for separating the collection's concerns from the user of the collection”

for loop becomes less useful

Don’t just use ForEach!

Code becomes more declarative

Declarative? “a statement specifies some aspect of the desired answer with no notion of how that answer is to be determined”
‘ The Lush Landscape of Languages’ - The ThoughtWorks Anthology
Rebecca Parsons

Requires a mental shift from imperative thinking

Transformational mindset
Patrick Logan in the comments section
http://www.markhneedham.com/blog/2010/01/20/functional-collectional-parameters-some-thoughts/

Current Input ??? ??? ??? Desired Output

Going from one collection to another

var words = new List<string> { “hello”, “world” }; var upperCaseWords = new List<string>(); foreach (var word in words) { upperCaseWords.Add(word.ToUpper()); }

a.k.a. map

“ hello”, “world” ???? “HELLO”, “WORLD”

“hello”, “world” Select “HELLO”, “WORLD”

var words = new List<string> { “hello”, “world” }; var upperCaseWords = words.Select(w => w.ToUpper());

Remove values we don’t want

var words = new List<string> {“hello”, “world”}; var wordsWithH = new List<string>(); foreach (var word in words) { if(word.Contains(“h”) wordsWithH.Add(word); }

a.k.a. filter

“ hello”, “world” ???? “hello”

“ hello”, “world” Where “hello”

var words = new List<string> {“hello”, “world”} ; var wordsWithH = words.Where(w => w.Contains(“h”));

Summing some values

var values = new List<int> { 1,2,3 }; var total = 0; foreach (var value in values) { total += value; }

a.k.a. reduce

1, 2, 3 ??? 6

1, 2, 3 Sum 6

var values = new List<int> { 1,2,3 }; var total = val ues.Sum(v => v);

Some examples from projects

Getting the first value that matches a criteria

Foo(“mark”, true), Foo(“dave”, false), Foo(“mike”, true) ???? Foo(“mark”, true)

Foo(“mark”, true), Foo(“dave”, false), Foo(“mike”, true) Where Foo(“mark”, true), Foo(“mike”, true) ??? Foo(“mark”, true)

Foo(“mark”, true), Foo(“dave”, false), Foo(“mike”, true) Where Foo(“mark”, true), Foo(“mike”, true) First Foo(“mark”, true)

var foos = new List<Foo> { new Foo(“mark”, true), new Foo(“dave”, false), new Foo(“mike”, true) }; var firstSpecialFoo = foos. Where(f => f.HasSpecialFlag()). First());

var foos = new List<Foo> { new Foo(“mark”, true), new Foo(“dave”, false), new Foo(“mike”, true) }; var firstSpecialFoo = foos.First(f => f.HasSpecialFlag());

Removing some columns from a dataset

a,b,c,d,e,f ???? “a,d,e,f”

a,b,c,d,e,f Where IEnumerable of a, d, e, f ??? “a,d,e,f”

a,b,c,d,e,f Where IEnumerable of a, d, e, f ??? String.Join on [a, d, e, f] “a,d,e,f”

a,b,c,d,e,f Where IEnumerable of a, d, e, f ToArray() String.Join on [a, d, e, f] “a,d,e,f”

var aRow = “a, b, c, d, e, f”; var newRow = String.Join(“,”, aRow .Split(‘,’) .Where((_, idx) => !(idx == 1 || idx == 2)) .ToArray());

var aRow = “a, b, c, d, e, f”; var rowWithColumnsRemoved = aRow .Split(‘,’) .Where((_, idx) => !(idx == 1 || idx == 2)) .ToArray()); var newRow = String.Join(“,”, rowWithColumnsRemoved);

Checking if any parameters are null

public class SomeObject { public SomeObject(string p1, string p2, string p3) { if (p1 == null ) throw new Exception(...); if (p2 == null ) throw new Exception(...); if (p3 == null ) throw new Exception(...); // rest of constructor logic } }

public class SomeObject { public SomeObject(string p1, string p2, string p3) { var params = new List<string> {p1, p2, p3}; if (params.Any(p => p == null) throw new Exception(...); // rest of constructor logic } }

Some lessons from the wild

Dealing with the null collection

public int SumNumbers(List< int > list)
{
if(list == null )
return 0;
return list.Sum(v => v);
}

public IEnumerable<T> EmptyIfNull( this IEnumerable<T> collection)
{
if(collection == null )
return new List<T>();
return collection;
}
Chris Ammerman in the comments section http://www.markhneedham.com/blog/2009/06/16/functional-collection-parameters-handling-the-null-collection/

public int SumNumbers(List< int > list)
{
return list.
EmptyIfNull().
Sum(v => v);
}

LINQ and the forgotten abstraction

Avoid passing lists around

public class Quote
{
public List<Coverage> Coverages
{
get; set;
}
}

Later on in the view…

<% var coverages = Model.Quote.Coverages; %>
<% if(coverages.Where(
c => c.Name == “coverageType1”) %>
...
...

Objects as the mechanism for encapsulation

public class Coverages
{
private readonly List<Coverage> coverages;
public Coverages(List<Coverage> coverages)
{
this.coverages = new List<Coverage>(coverages);
}
public Coverage CoverageType1
{
get
{
return coverages.Where(
c => c.Name == “coverageType1”;
}
}
}

LINQ is duplication too!

{
{
get
{
}
}
{
get
{
}
}
}

{
{
get { return CoverageByName(“coverageType1”); }
}
{
get { return CoverageByName(“coverageType2”); }
}
public Coverage CoverageBy(string name)
{
return coverages.Where(c => c.Name == name);
}
}

Don’t forget “extract method”

var someFoos = new List<Foo>()
// put some foos in the list
someFoos.Select(f => new NewFoo
{
Property1 = f.Property1
...
});

var someFoos = new List<Foo>()
// put some foos in the list
someFoos.Select(f => CreateNewFooFrom(f));

Naming lambda variables

Putting functions in maps

public void SomeMethodParsing( string input) { if (input == “input1”) { someMethod(input); } else if (input == “input2”) { someOtherMethod(input); } // and so on }

Dictionary< string , Action< string >> inputs = new Dictionary< string , Action< string >> { { “input1” , someMethod }, { “input2” , someOtherMethod } }; public void SomeMethodParsing( string input) { var method = inputs[input]; method(input); }

Large Medium Small

Using functions to simplify some GOF patterns

Action

public class SomeObject
{
private readonly IStrategy strategy;
public Foo(IStrategy strategy)
{
this.strategy = strategy;
}
public void DoSomething(string value)
{
strategy.DoSomething(value);
}
}

public class Strategy : IStrategy
{
{
// do something with string
}
}

public class SomeObject
{
private readonly Action<string> strategy;
public Foo(Action<string> strategy)
{
this. strategy = strategy;
}
{
strategy(value);
}
}

Need to ensure we don’t lose readability/understandability

The hole in the middle pattern
Brian Hurt
http://enfranchisedmind.com/blog/posts/the-hole-in-the-middle-pattern/

Common beginning and end. Only the middle differs

public class ServiceCache<Service>
{
protected Res FromCacheOrService
<Req, Res>(Func<Res> serviceCall, Req request)
{
var cachedRes = cache.RetrieveIfExists(
typeof(Service), typeof(Res), request);
if(cachedRes == null)
{
cachedRes = serviceCall();
cache.Add(typeof(Service), request, cachedRes);
}
return (Res) cachedRes;
}
}

public class CachedService : ServiceCache<IService>
{
public MyResult GetMyResult(MyRequest request)
{
return FromCacheOrService(
() => service.GetMyResult(request), request);
}
}

Other ideas I’m intrigued about

The option type

The nested closure

File.open(“someFile.txt”, ‘w’) do |out| out << “add something to file” end

public class MyFile { public static void Open(string filePath, Action<StreamWriter> block) { using(var writer = new StreamWriter()) { block(writer ); } } }

MyFile.Open(“c:ark.txt”, f => f.WriteLine(“some random text”));

Writing extension methods to create functional abstractions

Learning more

3 things to take away

The transformational mindset

Objects are still the mechanism for encapsulation

Simplify GOF design patterns by using functions

Thanks to…
Dave Cameron (not that one!) who I worked with on the structure and general content of the talk.
Mike Wagg, Brian Blignaut, Chris Owen for reviewing the talk and giving their input.

Questions?
Mark Needham
http://www.markhneedham.com/blog/
[email_address]
Twitter: markhneedham

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Mixing functional and object oriented approaches to programming in C#

by Mark Needham on Jan 30, 2010

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