OCaml while true do loop

Question

I have the following code:

let a = 1 in
while a<10 do
  let a = a+1 in
done
Printf.printf "the number is now %d\n" a

The interpreter is complaining about line 4, which is done and I have no idea what is wrong here.
I understand that OCaml is a functional language and the variables are immutable. I should not try to change the value of a here. But still, there is a while true do .. done loop in OCaml. I hope you get the idea of what I am trying to do here. How shall I modify the code to do this job with while true do .. done ?
I am very new to functional programming. Please teach me the right way to get started with it. I find myself stuck in the deadlock of thinking imperatively.

Answer 1

The let ... in construct expects another expression behind. You can, for example use the () value (which basically means "nothing")

So the code

let a = 1 in
while a<10 do
   let a = a+1 in
   ()
done
Printf.printf "the number is now %d\n" a

It will compile. But it will loop indefinitely because the a defined as 1 at start is different than the a declared as a+1. Both are constant different values on different scopes, and a declaration inside the body of a while is limited to that occurence of the body.

You may get what you want by specifying a as mutable using the ref function and its handlers:

let a = ref 1 in
while !a < 10 do
 a := !a + 1
done
Printf.printf "the number is now %d\n" !a

Note that you loose all the benefits of FP by using a while loop and mutable values.

To do it in a functionnal manner, you can use a recursive function:

let rec f a =
 if a < 10
 then f (a+1)
 else a
in
let a = f 1 in
Printf.printf "the number is now %d\n" a

This one is the true right manner to do the job. If you want to do FP, avoid at all costs to use a while loop.