F# development and unit testing?

Question

I just got started with F#, which is my first functional language. I have been working quasi-exclusively with C#, and enjoy a lot how F# leads me to re-think how I write code. One aspect I find a bit disorienting is the change in the process of writing code. I have been using TDD for years in C# now, and really appreciate to have unit tests to know where I am at.

So far, my process with F# has been to write some functions, play with them with the interactive console until I am "reasonably" sure they work, and tweak & combine. This works well on small-scale problems like the Euler Project, but I can't imagine building something large that way.

How do people approach unit testing and building a test suite for a F# program? Is there an equivalent to TDD? Any pointers or thoughts are appreciated.

Answer 1

Test-driven developers should feel right at home in functional languages like F#: small functions that give deterministically repeatable results lend themselves perfectly to unit tests. There are also capabilities in the F# language that facilitate writing tests. Take, for example, Object Expressions. You can very easily write fakes for functions that take as their input an interface type.

If anything, F# is a first-class object-oriented language and you can use the same tools and tricks that you use when doing TDD in C#. There are also some testing tools written in or specifically for F#:

• NaturalSpec
• FsCheck
• FsTest
• FsUnit

Matthew Podwysocki wrote a great series on unit testing in functional languages. Uncle Bob also wrote a thought provoking article here.

Answer 2

I use NUnit, and it doesn't strike me as hard to read or onerous to write:

open NUnit.Framework

[<TestFixture>]
type myFixture() = class

    [<Test>]
    member self.myTest() =
       //test code

end

Since my code is a mix of F# and other .Net languages, I like the fact that I write the unit tests in basically the same fashion and with similar syntax in both F# and C#.

Answer 3

Have a look at FsCheck, an automatic testing tool for F#, is basically a port of Haskell's QuickCheck. It allows you to provide a specification of the program, in the form of properties that the functions or methods should satisfy, and FsCheck tests that the properties hold in a large number of randomly generated cases.

FsCheck CodePlex Page

FsCheck Author Page

Answer 4

I think this is a very interesting question that I have wondered about a lot myself. My thoughts so far are only thoughts, so take them for what they are.

I think that the safety net of an automated test suite is too valuable an asset to let go, however alluring that interactive console may be, so I plan to continue writing unit tests as I've always done.

One of the main strengths of .NET is the cross-language capabilities. I know I'm going to be writing F# production code soon, but my plan is to write unit tests in C# to ease my way into what is for me a new language. In this way, I also get to test that what I write in F# will be compatible with C# (and other .NET languages).

With this approach, I understand that there are certain features of F# that I can only use internally in my F# code, but not expose as part of my public API, but I will accept that, just as I accept today that there are certain things C# allows me to express (like uint) that aren't CLS compliant, and so I refrain from using them.

Answer 5

As dglaubman suggests you can use NUnit. xUnit.net also provides support for this and works well with TestDriven.net. The code looks similar to NUnit tests but without the requirement to wrap the test in a containing type.

#light

// Supply a module name here not a combination of module and namespace, otherwise
// F# cannot resolve individual tests nfrom the UI.
module NBody.DomainModel.FSharp.Tests

open System
open Xunit

open Internal

[<Fact>]
let CreateOctantBoundaryReordersMinMax() =
    let Max = VectorFloat(1.0, 1.0, 1.0)
    let Min = VectorFloat(-1.0, -1.0, -1.0)

    let result = OctantBoundary.create Min Max

    Assert.Equal(Min, result.Min)     
    Assert.Equal(Max, result.Max) 

Answer 6

You could have a look at FSUnit - though I haven't used it yet, it might worth a try. Certainly better than using for example (native) NUnit in F#.

Answer 7

Despite being a bit late to the party, I'd like to welcome Mathias to F# (better late than never ;)) and chime in that you might like my unit testing library, Expecto

Expecto has some features you might like:

• F# syntax throughout, tests as values; write plain F# to generate tests
• Use the built-in Expect module, or an external lib like Unquote for assertions
• Parallel tests by default
• Test your Hopac code or your Async code; Expecto is async throughout
• Pluggable logging and metrics via Logary Facade; easily write adapters for build systems, or use the timing mechanism for building an InfluxDB+Grafana dashboard of your tests' execution times
• Built in support for BenchmarkDotNet
• Build in support for FsCheck; makes it easy to build tests with generated/random data or building invariant-models of your object's/actor's state space

--

open Expecto

let tests =
  test "A simple test" {
    let subject = "Hello World"
    Expect.equal subject "Hello World" "The strings should equal"
  }

[<EntryPoint>]
let main args =
  runTestsWithArgs defaultConfig args tests

https://github.com/haf/expecto/