In support of the article, your code would still throw an exception if all the customers in the list have missing credit. https://github.com/dotnet/fsharp/blob

In support of the article, your code would still throw an exception if all the customers in the list have missing credit. https://github.com/dotnet/fsharp/blob/master/src/fsharp/FSharp.Core/list.fs#L627 https://github.com/dotnet/fsharp/blob/master/src/fsharp/FSha... You could instead write it as the following to handle that. let average (customers : Customer list) = customers |> List.choose (fun c -> c.Credit) |> function | [] -> Error "no customers with credit" | xs -> Ok (List.average xs)