F# – Control Flow

There are 3 main loop types in F# for imperative programming style:

• Iteration
  for id = … to … do
• Sequence loop
  for … in expression do
• Pre-test loop
  while … do

This is an example code for each loops:

#light

/// Iteration
printfn "for to do"
printf "["
for i=1 to 5 do
    printf "%d " i
printf "]"

/// Sequence loop
printfn ""
printfn ""
printfn "for in do"
let items = seq{1..5}
printf "["
for i in items do
    printf "%d " i
printf "]"

/// Pre-test loop
printfn ""
printfn ""
printfn "while do"
printf "["
let mutable i = 1
while i<=5 do 
    printf "%d " i
    i <- i+1
printf "]"

for to do [1 2 3 4 5 ] for in do [1 2 3 4 5 ] while do [1 2 3 4 5 ]

All the loop types can be found in C#.

seq{1..5} is a sequence data type that means:

val it : seq<int> = seq [1; 2; 3; 4; ...]

So we can say it contains 1,2,3,4,5.

The

let mutable i = 1

line means variable i can change its value. So i <- i+1 means assignment and can be evaluated.