Pure Functions

Pure Functions

In this lesson — part of Functional Foundations — you'll learn pure functions in F# and why it matters in real work.

Why it matters

Functional style — pure functions and immutability — makes code easier to test and reason about.

Key ideas

• Pure functions
• Immutability
• No shared mutable state
• Predictable outputs

In practice

Here's how it looks in idiomatic F#:

// Pure, immutable, expression-oriented core
let inc x = x + 1                       // pure: no side effects

// Option replaces null; pattern match to handle absence
let tryHead = function [] -> None | h :: _ -> Some h

let describe xs =
    match tryHead xs with
    | Some h -> $"first is {h}"
    | None   -> "empty"

printfn "%d %s" (inc 4) (describe [10; 20])

F# note: Option<'T> is F#'s null-free way to represent absence, and exhaustive matching forces you to handle the None case at compile time rather than risking a NullReferenceException.

Try it yourself

Exercise: In F#, rewrite a function that mutates a global so it's pure.

Recap

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