Functions

Functions are defined with let, just like values. Parameters are separated by spaces, not commas, and so are the arguments when you call it:

let square x = x * x
let add a b = a + b
let greet name = printfn "Hello, %s!" name

printfn "%d" (square 6)
printfn "%d" (add 2 3)
greet "Ada"

36
5
Hello, Ada!

• There's no return keyword: a function returns the value of its last expression.
• Wrap a call in parentheses when it's an argument to another function: printfn "%d" (square 6). Without them, F# would read square and 6 as two separate arguments to printfn.

Multi-line functions

let bmi weightKg heightM =
    let value = weightKg / (heightM * heightM)
    let category =
        if value < 18.5 then "underweight"
        elif value < 25.0 then "normal"
        else "overweight"
    sprintf "%.1f (%s)" value category

printfn "%s" (bmi 70.0 1.75)

22.9 (normal)

Local values like value and category exist only inside the function.

Signatures

In F# Interactive, a function's type shows its parameters and result separated by arrows:

val square: x: int -> int
val add: a: int -> b: int -> int
val bmi: weightKg: float -> heightM: float -> string

Read int -> int -> int as "takes an int, then an int, and returns an int". The next lessons explain why it's written as a chain.

Recursion needs rec

A function that calls itself must be marked rec:

let rec factorial n =
    if n <= 1 then 1
    else n * factorial (n - 1)

let rec countdown n =
    if n > 0 then
        printf "%d " n
        countdown (n - 1)
    else
        printfn "liftoff!"

printfn "%d" (factorial 5)
countdown 3

120
3 2 1 liftoff!

Functions with no parameters

A function that takes no input takes unit:

let sayHi () = printfn "Hi!"
sayHi ()
sayHi ()

Hi!
Hi!

Without the (), let sayHi = printfn "Hi!" would be a value: it would print once, immediately, and never again.

Practice

Write a function isLeapYear year (divisible by 4, except centuries, unless divisible by 400) and print the result for 1900, 2000, and 2024.

let isLeapYear year =
    (year % 4 = 0 && year % 100 <> 0) || year % 400 = 0

printfn "%b %b %b" (isLeapYear 1900) (isLeapYear 2000) (isLeapYear 2024)

false true true