Data Types

Every value in F# has a type, known at compile time. The everyday types:

let count = 42
let big = 9_000_000_000L
let ratio = 0.75
let price = 19.99m
let isReady = true
let grade = 'A'
let greeting = "hello"

printfn "%A %A %A %A %A %A %A" count big ratio price isReady grade greeting
printfn "%s" (count.GetType().Name)
printfn "%s" (price.GetType().Name)

42 9000000000L 0.75 19.99M true 'A' "hello"
Int32
Decimal

%A shows each value the way you'd write it in code — note the L, M, and quotes. Underscores in number literals are just for readability. GetType().Name shows the underlying .NET type: F#'s int is .NET's Int32.

Converting between types

F# never converts automatically. Use the conversion functions, which are named after the target type:

let a = 7
let b = 2.5

printfn "%f" (float a + b)
printfn "%d" (a + int b)
printfn "%d" (int "123" + 1)
printfn "%s" (string a + "!")
printfn "%M" (decimal a / 3m)
printfn "%c" (char 65)
printfn "%d" (int 'A')

9.500000
9
124
7!
2.3333333333333333333333333333
A
65

int 2.5 truncates toward zero, and int "abc" throws an exception — you'll learn safe parsing with TryParse later.

unit

unit has exactly one value, written (). Functions that only do something (like printing) return unit, the way void works in C-style languages — except unit is a real value you can pass around:

let result = printfn "side effect!"
printfn "%A" result

side effect!
()

Common mistakes

• Writing 3 / 2 and expecting 1.5 — both are int, so the answer is 1. Write 3.0 / 2.0.
• Mixing int and float in arithmetic (1 + 2.0) — a compile error; convert one side.
• Using float for money — use decimal (19.99m) to avoid rounding errors.