Why and What to Test

Why and What to Test

In this lesson — part of Testing Fundamentals — you'll learn why and what to test in F# and why it matters in real work.

Why it matters

Tests prove your code works and keep it working as you change it.

Key ideas

• What to test
• Arrange-Act-Assert
• Running a test suite
• Good vs. brittle tests

In practice

Here's how it looks in idiomatic F#:

// Using Xunit + FsUnit-style assertions
open Xunit

let add x y = x + y

[<Fact>]
let ``add returns sum`` () =
    Assert.Equal(5, add 2 3)

[<Theory>]
[<InlineData(1, 2, 3)>]
[<InlineData(-1, 1, 0)>]
let ``add is correct for cases`` (a, b, expected) =
    Assert.Equal(expected, add a b)

F# note: F# lets you name tests with double-backtick identifiers (` add returns sum `) for readable reports, and property-based testing via FsCheck is especially idiomatic given how easy pure functions are to test.

Try it yourself

Exercise: In F#, write three tests for a function that reverses a string.

Recap

You now understand why and what to test and can apply it in F#. Mark this lesson complete and continue to the next one.