Background

The expression problem is an ideal to strive towards in the design of programming languages. It is relevant when the data in your program can be represented as cases, with operations that operate on those cases. For example, say your data represents shapes. The cases would be circle, rectangle, triangle, etc. Some possible operations would be area(), perimeter(), centroid(), etc.

The expression problem is solved in a language when you can add a new case or a new operation without needing to recompile any of the other cases/operations. The problem is stated by Philip Wadler (who coined the term) as follows:

The Expression Problem is a new name for an old problem. The goal is to define a datatype by cases, where one can add new cases to the datatype and new functions over the datatype, without recompiling existing code, and while retaining static type safety (e.g., no casts).

The problem is best understood via code, so we will examine the shapes example outlined above in both C# and F#. Object oriented and functional programming languages have different characteristics within the context of the expression problem.

Object Oriented - C#

The way that we would typically represent cases that support common operations in object oriented programming is to create an abstract superclass that defines virtual/abstract methods, and for each case create a subclass that overrides those methods. For example, here is our abstract superclass. It defines the operations that our cases will support:

Shape.cs public abstract class Shape { abstract public double Area(); abstract public double Perimeter();}

Here is each case, defined in their own subclass:

Rectangle.cs pubic class Rectangle : Shape{ private readonly double width; private readonly double height; public Rectangle( double width, double height) { this .width = width; this .height = height; } public override double Area() { return this .width * this .height; } public override double Perimeter() { return 2 * ( this .width + this .height); }}

Circle.cs public class Circle : Shape{ private readonly double radius; public Circle( double radius) { this .radius = radius; } public override double Area() { return Math.PI * this .radius * this .radius; } public override double Perimeter() { return 2 * Math.PI + this .radius; }}

What if we wanted to create another case? It’s as simple as adding another subclass of Shape. For example, let’s add the case of a RightTriangle shape:

Triangle.cs public class RightTriangle : Shape{ private readonly double baseLength; private readonly double height; public RightTriangle( double baseLength, double height) { this .baseLength = baseLength; this .height = height; } public override double Area() { return 0.5 * this .baseLength * this .height; } public override double Perimeter() { return this .baseLength + this .height + Math.Sqrt( this .baseLength * this .baseLength + this .height * this .height); }}

The case-adding clause of the expression problem was satisfied, because we didn’t need to modify the base class or any of the other subclasses to add a new case. What about adding a new operation over these cases? Let’s add centroid (the center of the shape). We’ll have to add it to our base class first:

Shape.cs public abstract class Shape { abstract public double Area(); abstract public double Perimeter(); abstract public Tuple< double , double > Centroid();}

Oh no! We’re going to have to add it to every single one of our subclasses.

C# public class Rectangle : Shape{ private readonly double width; private readonly double height; public Rectangle( double width, double height) { this .width = width; this .height = height; } public override double Area() { return this .width * this .height; } public override double Perimeter() { return 2 * ( this .width + this .height); } public override Tuple< double , double > Centroid() { return Tuple.Create( this .width / 2 , this .height / 2 ); }} public class Circle : Shape{ private readonly double radius; public Circle( double radius) { this .radius = radius; } public override double Area() { return Math.PI * this .radius * this .radius; } public override double Perimeter() { return 2 * Math.PI + this .radius; } public override Tuple< double , double > Centroid() { return Tuple.Create( this .radius, this .radius); }} public class RightTriangle : Shape{ private readonly double baseLength; private readonly double height; public RightTriangle( double baseLength, double height) { this .baseLength = baseLength; this .height = height; } public override double Area() { return 0.5 * this .baseLength * this .height; } public override double Perimeter() { return this .baseLength + this .height + Math.Sqrt( this .baseLength * this .baseLength + this .height * this .height); } public override Tuple< double , double > Centroid() { return Tuple.Create( 2 * this .baseLength / 3 , this .height / 3 ); }}

This fails the expression problem, because adding new operations will force us to recompile the code of already existing cases.

So we can see that in an object oriented programming style when using inheritance to represent our cases, it is easy to add new cases ( RightTriangle ) but difficult to add new operations ( Centroid ). Let’s examine the functional style of programming via F#, to see what characteristics it has when posed with this problem.

Functional - F#

In functional programming with Algebraic Data Types, cases are conveniently represented using sum types. In F# these are implemented as discriminated unions. To operate on these cases, you must have a match expression which specifies the desired output for each case. One very nice feature of discriminated unions and match expressions, is that they support exhaustive matching. This means that if you don’t handle all of the cases, you will get a compiler error. This leads to more correct code, as all combinations of cases must be explicitly accounted for. Here is a type definition for our shapes, along with some functions that provide operations on that data:

Shape.fs open Systemtype Shape = | Rectangle of width : double * height : double | Circle of radius : doublelet area shape = match shape with | Rectangle ( width , height ) -> width * height | Circle radius -> Math.PI * radius * radiuslet perimeter shape = match shape with | Rectangle ( width , height ) -> 2 . 0 * ( width + height ) | Circle radius -> 2 . 0 * Math.PI * radius (* Test out some code *) let circle = Circle 2 . 42area circle |> Console.WriteLine

If you’ve never used F# before, you’ll notice how much more succinct it is than the equivalent C#. The match expressions look like super-charged switch statements. Each case in the shape example is represented by a case in our discriminated union, and each operation is represented by a function. Let’s try what was so difficult in C#; let’s try to add a new operation, centroid .

F# let centroid shape = match shape with | Rectangle ( width , height ) -> 0 . 5 * width , 0 . 5 * height | Circle radius -> radius , radius (* Test out some code *) let rectangle = Rectangle ( 42 . 42 , 67 . 67 ) centroid rectangle |> printfn "%A"

We didn’t have to alter any of the other functions, or the initial definition of the cases. Neat! So, the expression problem is satisfied for creating new operations in the functional style of programming. This was something we couldn’t achieve a second ago with idiomatic object oriented code. What if we try to add a new case to our functional code:

F# type Shape = | Rectangle of width : double * height : double | Circle of radius : double | RightTriangle of base' : double * height : double

If we were to recompile, we would see that all of our match expressions now fail to build. This is because of the exhaustive pattern matching previously mentioned. We are now missing the RightTriangle case in our match expressions. Consequently, we must edit all of the functions to account for the new case:

Shape.fs open Systemtype Shape = | Rectangle of width : double * height : double | Circle of radius : double | RightTriangle of base' : double * height : doublelet area shape = match shape with | Rectangle ( width , height ) -> width * height | Circle radius -> Math.PI * radius * radius | RightTriangle ( base' , height ) -> 0 . 5 * base' * heightlet perimeter shape = match shape with | Rectangle ( width , height ) -> 2 . 0 * ( width + height ) | Circle radius -> 2 . 0 * Math.PI * radius | RightTriangle ( base' , height ) -> base' + height + Math.Sqrt ( base' * base' + height * height ) let centroid shape = match shape with | Rectangle ( width , height ) -> 0 . 5 * width , 0 . 5 * height | Circle radius -> radius , radius | RightTriangle ( base' , height ) -> 2 . 0 * base' / 3 . 0 , height / 3 . 0let triangle = RightTriangle ( 42 . 42 , 67 . 67 ) centroid triangle |> printfn "%A"

The thing that was easy in object oriented code (adding new cases), was difficult in functional code. The thing that was easy in functional code (adding new operations), was difficult in object oriented code. This is the crux of the expression problem, designing programming language features that can do both things well. If you have a data structure that won’t be getting new cases added very frequently (or ever), the functional approach is better. If you have a data structure that will be constantly adding new cases but has less frequently added operations, the object oriented style is better.

Using the Visitor Pattern

The Visitor Pattern confers some of the benefits of the functional style in an object oriented language. It allows you to add new operations to your data structure, without having to recompile the code that defines the data structure itself. The downsides are, it’s hard to understand if you don’t know what you’re looking at, and it clutters your code with implementation details that aren’t really relevant to your business logic.

Here is a basic implementation of the Visitor Pattern using our shapes example above (with Rectangle , Circle , and Area ):

C# using System; using System.Collections.Generic; interface IVisitable { void Accept(IVisitor visitor); } public interface IVisitor { void Visit(Rectangle rectangle); void Visit(Circle circle);} public abstract class Shape : IVisitable{ abstract public void Accept(IVisitor visitor);} public class Rectangle : Shape{ public double Width { get ; private set ; } public double Height { get ; private set ; } public Rectangle( double width, double height) { this .Width = width; this .Height = height; } public override void Accept(IVisitor visitor) { visitor.Visit( this ); }} public class Circle : Shape{ public double Radius { get ; private set ; } public Circle( double radius) { this .Radius = radius; } public override void Accept(IVisitor visitor) { visitor.Visit( this ); }} public class AreaVisitor : IVisitor{ public double Area { get ; private set ; } public void Visit(Rectangle rectangle) { this .Area = rectangle.Width * rectangle.Height; } public void Visit(Circle circle) { this .Area = Math.PI * circle.Radius * circle.Radius; }} public class Program { public static void Main() { var rectangle = new Rectangle( 1.1 , 2.2 ); var circle = new Circle( 1 ); var shapeList = new List<Shape>() { rectangle, circle }; foreach ( var shape in shapeList) { var areaVisitor = new AreaVisitor(); shape.Accept(areaVisitor); Console.WriteLine(areaVisitor.Area); } }}

Our data structures and operations are separated using this design pattern. All of the area calculations are grouped inside of the AreaVisitor class (rather than a method on each class itself). One negative effect of this pattern as implemented here is the breaking of encapsulation of our shape subclasses. Previously, we could hide data private to the specific shapes. For example, a circle stored its radius using the following signature: private readonly double radius; . No code outside of the class could access its value. In the new version, our AreaVisitor needs access to this data to calculate the area, so it is declared as: public double Radius { get; private set; } .

What if we need to add an operation to our code? For example, let’s add perimeter calculation code:

C# public class PerimeterVisitor : IVisitor{ public double Perimeter { get ; private set ; } public void Visit(Rectangle rectangle) { this .Perimeter = 2 * (rectangle.Width + rectangle.Height); } public void Visit(Circle circle) { this .Perimeter = 2 * Math.PI + circle.Radius; }}

All we had to do was add another Visitor class implementing the perimeter logic. The visitor pattern satisfies the operations condition of the expression problem. What if we want to add another shape? First we need to add another shape subclass:

C# public class RightTriangle : Shape{ public double BaseLength { get ; private set ; } public double Height { get ; private set ; } public RightTriangle( double baseLength, double height) { this .BaseLength = baseLength; this .Height = height; } public override void Accept(IVisitor visitor) { visitor.Visit( this ); }}

We’ve had our RightTriangle subclass implement the IVisitable interface to be compatible with our visitor pattern. Now we need to update the IVisitor interface to add the new case. This will force the different visitor implementations to account for our new case.

C# public interface IVisitor { void Visit(Rectangle rectangle); void Visit(Circle circle); void Visit(RightTriangle rightTriangle);}

Here are the visitor implementations updated:

C# public class AreaVisitor : IVisitor{ public double Area { get ; private set ; } public void Visit(Rectangle rectangle) { this .Area = rectangle.Width * rectangle.Height; } public void Visit(Circle circle) { this .Area = Math.PI * circle.Radius * circle.Radius; } public void Visit(RightTriangle rightTriangle) { this .Area = 0.5 * rightTriangle.BaseLength * rightTriangle.Height; }} public class PerimeterVisitor : IVisitor{ public double Perimeter { get ; private set ; } public void Visit(Rectangle rectangle) { this .Perimeter = 2 * (rectangle.Width + rectangle.Height); } public void Visit(Circle circle) { this .Perimeter = 2 * Math.PI + circle.Radius; } public void Visit(RightTriangle rightTriangle) { this .Perimeter = rightTriangle.BaseLength + rightTriangle.Height + Math.Sqrt(rightTriangle.BaseLength * rightTriangle.BaseLength + rightTriangle.Height * rightTriangle.Height); }}

If we add a new case, we have to update every single visitor class to account for the new case. This fails the cases condition of the expression problem, because we would have to recompile the code implementing the visitor classes. The visitor pattern reproduces the characterstics of the functional programming approach in an object oriented language.

Conclusion

Faced with such a difficult problem, what is to be done in .NET? Ultimately it depends on making compromises based on how your code evolves. If your data remains the same over time, the functional or visitor pattern approach is best. It allows you to add new operations over your data (as functions or visitor classes) without having to recompile the rest of your application. If your data is constantly changing with new cases being added all the time, the object oriented approach may be best. It allows you to add new cases (as subclasses) without recompiling the rest of your application. As with many things in programming, the answer is “it depends”.