For

In F# a loop continues forward—we implement loops with for and while. Loops are powerful, but functional-based designs are usually emphasized in this language.

In this language, declarative constructs are often preferred. But for getting things done, loops are often a good choice—and they can be rewritten later if needed.

For example

This program shows 2 for-loops, and both of the constructs loop over the same values. In programs, either form can be used.

// Loop from 0 through 3 inclusive.
// ... The variable i is incremented by 1 each time.
for i = 0 to 3 do
    printfn "For, to: %A" i

// This is the same loop but with a range.
for i in 0 .. 3 do
    printfn "For, in: %A" i
For, to: 0
For, to: 1
For, to: 2
For, to: 3
For, in: 0
For, in: 1
For, in: 2
For, in: 3

For-in, list

A for-in loop enumerates over each element in a collection (like a list). Here we create a list of ints called "codes."

let codes = [3; 12; 34; 3]

// Loop over ints from list.
for code in codes do
    // If code is 3 then write a special word.
    if (code = 3) then
        printfn "Three"

    // Write value of int from list.
    printfn "%A" code
Three
3
12
34
Three
3

Downto

With this keyword we create a decrementing loop. After each iteration, the iteration variable (in this program "i") is reduced by 1.

// Specify a decrement for-loop with downto.
for i = 10 downto 6 do
    printfn "%A" i
10
9
8
7
6

While

The most versatile loop in F# is the while-loop. Here we can increment, decrement, or loop forever. We must be careful not to make mistakes.

// Use an iteration variable that starts at 4.
// ... The variable must be mutable.
let mutable x = 4

// Continue looping while x is positive.
while x >= 0 do
    // Write value.
    printfn "%A" x
    // Decrement x.
    x <- x - 1
4
3
2
1
0

In functional programs, recursion and method-based iteration is often preferred. But loops are still wonderful. In F# we can use iteration and increment variables.