Mocking F#

The intent of this entry is to show how to do TDD (Test Driven Development) in F# with the help of a dynamic mock object framework such as RhinoMocks. Along the way, we will explore how to implement an object oriented design in F# with features including namespaces, classes, interfaces, properties, and members. This example includes several best practices, but is by no means production ready. Most specifically, it is missing error handling and validation. In addition, certain patterns and practices have been intentionally ignored in order to make the example a little easier to understand.

This is an implementation of a simplified customer service that might exist in any standard LOB (Line of Business) application. In the end, we have a service that allows the current balance of a customer to be returned with a specified discount applied.

As always, we write the tests first (Note: As is my usual style, the tests are in C# and the code is in F#):

C# Customer Entity Test:

[TestMethod]
public void CanCreateCustomer()
{
    FSharpMockExample.Entities.ICustomer customer = new FSharpMockExample.Entities.Customer(1, "ABC, Corp.", 20);
    Assert.AreEqual(customer.Id, 1);
    Assert.AreEqual(customer.Name, "ABC, Corp.");
}

[TestMethod]
public void CanCalculateBalanceWith10PercentDiscount()
{
    FSharpMockExample.Entities.ICustomer customer = new   FSharpMockExample.Entities.Customer(1, "ABC, Corp.", 20);
    decimal newBalance = customer.CalculateBalanceWithDiscount(.1M);
    Assert.AreEqual(newBalance, 18);
}

F# Customer Entity:

Signature File:

#light
namespace FSharpMockExample.Entities

type ICustomer = interface
    abstract Id: int with get
    abstract Name: string with get
    abstract CalculateBalanceWithDiscount: decimal -> decimal
end

type Customer = class
    new: int*string*decimal -> Customer
    interface ICustomer 
end

Source File:

#light
namespace FSharpMockExample.Entities

type ICustomer = interface
    abstract Id: int with get
    abstract Name: string with get
    abstract CalculateBalanceWithDiscount: decimal -> decimal
end

type Customer = class
    val id: int
    val name: string
    val balance: decimal
    new(id, name, balance) = 
        {id = id; name = name; balance = balance}
    interface ICustomer with
        member this.Id 
            with get () = this.id
        member this.Name
            with get () = this.name
        member this.CalculateBalanceWithDiscount discount =
            this.balance - (discount * this.balance)
    end
end

C# Customer Data Access Object Test:

[TestMethod]
public void CanGetCustomerById()
{
    FSharpMockExample.Data.ICustomerDao customerDao = new FSharpMockExample.Data.CustomerDao();
    FSharpMockExample.Entities.ICustomer customer = customerDao.GetById(1);
    Assert.AreEqual(customer.Id, 1);
    Assert.AreEqual(customer.Name, "ABC Company");
}

F# Customer Data Access Object:

Signature File:

#light
namespace FSharpMockExample.Data
open FSharpMockExample.Entities

type ICustomerDao = interface
    abstract GetById: int -> ICustomer
end

type CustomerDao = class
    new: unit -> CustomerDao
    interface ICustomerDao
end

Source File:

#light
namespace FSharpMockExample.Data
open FSharpMockExample.Entities

type ICustomerDao = interface
    abstract GetById: int -> ICustomer
end

type CustomerDao = class
    new()={}
    interface ICustomerDao with
        member this.GetById id =
            new Customer(id, "ABC Company", 20.00M) :> ICustomer
end

C# Customer Service Test:

[TestMethod]
public void CanCalculateBalance()
{
    var mocks = new MockRepository();
    var customerDao = mocks.CreateMock<FSharpMockExample.Data.ICustomerDao>();
    FSharpMockExample.Entities.ICustomer customer = new FSharpMockExample.Entities.Customer(1, "XYZ Company", 50);

    using (mocks.Record())
    {
        Expect.Call(customerDao.GetById(1)).IgnoreArguments().Return(customer);
    }

    using (mocks.Playback())
    {
        int customerId = 1;
        decimal discount = .1M;
        FSharpMockExample.Services.ICustomerService customerService = new FSharpMockExample.Services.CustomerService(customerDao);
        var balanceWithDiscount = customerService.CalculateBalaceWithDiscount(customerId, discount);

        Assert.AreEqual(balanceWithDiscount, 45);
    }
}

F# Customer Service:

Signature File:

#light
namespace FSharpMockExample.Services
open FSharpMockExample.Data
open FSharpMockExample.Entities

type ICustomerService = interface
    abstract CalculateBalaceWithDiscount: int*decimal -> decimal
end

type CustomerService = class
    new: unit -> CustomerService
    new: ICustomerDao -> CustomerService 
end

Source File:

#light
namespace FSharpMockExample.Services
open FSharpMockExample.Data
open FSharpMockExample.Entities

type ICustomerService = interface
    abstract CalculateBalaceWithDiscount: int*decimal -> decimal
end

type CustomerService = class
    val customerDao: ICustomerDao
    new (customerDao) = 
        {customerDao = customerDao}
    new () = 
        {customerDao = new CustomerDao()}
    interface ICustomerService with
        member this.CalculateBalaceWithDiscount (customerId, discount) =
            let customer = 
                this.customerDao.GetById(customerId)
            customer.CalculateBalanceWithDiscount(discount)    
    end
end

Hopefully, this example will inspire you to go out an give this amazing language a try.