Question

I’d like to know how to convert this code line by line from C# to F#. I am not looking to use any kind of F#’s idioms or something of the like. I am trying to understand how to map directly C#’s constructs to F#.

Here is the C# code:

//requires l.Length > 0
int GetMinimumValue(List<int> l) {
    int minVal = l[0];

    for (int i = 0; i < l.Length; ++i) {
        if (l[i] > minValue) {
            minVal = l[i];
        }
    }

    return minVal;
}

And here is my F# attempt:

let getMinValue (l : int list) =
    let minVal = l.Head

    for i = 0 to (l.Length-1) do
        if (l.Item(i) > minVal) then
            minVal = col.Item(i)

    minVal

Now, this ain’t working. The problem seems to be related with the minVal = col.Item(i) line:

This expression was expected to have type     unit     but here has type     bool

What is the problem, really?

Answer 1

If you want to convert it line by line then try the following

let getMinValue (l:System.Collections.Generic.List<int>) =
  let mutable min = l.Item(0)
  for i = 0 to (l.Count-1) do
    if l.Item(i) < min then min <- l.Item(i)
  min

Now as to why you’re getting that particular error. Take a look at the following line

minVal = col.Item(i)

In F# this is not an assignment but a comparison. So this is an expression which produces a bool value but inside the for loop all expressions must be void/unit returning. Hence you receive an error.

Assignment in F# has at least 2 forms that I am aware of.

// Assigning to a mutable value
let mutable v1 = 42
v1 <- 13

// Assigning to a ref cell
let v1 = ref 0
v1 := 42

And of course, you should absolutely read Brian’s article on this subject. It’s very detailed and goes over many of the finer points on translating between the two languages

• http://lorgonblog.spaces.live.com/Blog/cns!701679AD17B6D310!725.entry