return value to break function

Question

I am completely new to F# (started using it today) and relatively new to functional programming (I have minor experience with Lisp). I want to exit a function by returning a value when a certain condition is met so that the rest of the loop is not executed. Here is a C# illustration of what I want to do:

bool CheckRow (int n, int i)
{
    for(int j = 0; j < 9; j++)
        if (n == sudoku[i][j])
            return false;

    return true;
}

I tried implementing the same function in F# like this (sudoku is an array2D):

let CheckRow (n : int) (i : int) : bool = 

    for j = 0 to 8 do
        if (n = sudoku.[i, j]) then
            false

    true

However, I get the following error at false within the if: "This expression was expected to have type unit but here has type bool". What is the proper way to "return" from within a F# function?

Possible duplicate of F# returning boolean value in function — Callum Linington
I would assume here that you aren't doing this in an idiomatic way, I think you could use the seq type to help you here and do it a more functional way — Callum Linington
If you think you need to "exit early" out of an F# function, that's usually a clue that you're thinking about the problem wrong. Here, you want to see if there is any value in the row that fulfills a certain condition. That's what the Array.exists function is for. (Or List.exists, or Seq.exists -- see Bartek Kobylecki's answer). — rmunn
Control Flow and the whole series is a nice and quick overview. — s952163
Two resources that you should know about if you're just getting started with F#: F# for Fun and Profit is the best intro to F# anywhere on the Net, and Exercism is an excellent collection of F# exercises, roughly in order from easy to hard, to help ease you into thinking with functions. — rmunn

Asik Asik · Accepted Answer · 2016-11-22T19:24:29

Higher-order functions are nice of course, but at some point someone has to write a loop (e.g. to implement the higher-order function), and that'll eventually be you, so it's nice to know how to write loops in F#. There is no early return from a for loop in F#, but other types of loops do allow this:

// While loop, imperative style
let checkRow n i =
    let mutable clear = true
    let mutable j = 0
    while clear && j < 9 do
        clear <- n <> sudoku.[i, j]
        j <- j + 1
    clear

// Tail-recursive style - more idiomatic F#
let checkRow n i =
    let rec loop j =
        if j = 9 then true
        elif sudoku.[i, j] = n then false
        else loop (j + 1)
    loop 0

return value to break function

2 Answers