Question

Take the following example,

I have a class

public class SomeItem
{
    public string Name;
    public DateTime Published;
    public uint16 Size;
}

I have a List<SomeItem> and I want to calculate the total size of all the items.
In C# I’d simply write

var totalSize = items.Sum((i) => i.Size);

I have taken a look at the List functions in F# but they always complain about the types.

How would you write this in F#?

(I have tried the search engines, but search engine support for F# is terrible)

Answer 1

Assuming you have a value of type IEnumerable<Item>, you can use sum_by from the Seq module:

let totalSize = items |> Seq.sum_by (fun (i : Item) -> i.Size)

Note that F#’s Microsoft.FSharp.Collections.List<T> type is not the same class as the one you might be used to from System.Collections.Generic.List<T>, and they’re not interchanegable. However, the methods in the Seq module work on any IEnumerable<T>, and IEnumerable<T> is the same between F# and C#.