Question

So I’ve been reading the Expert F# book by Apress, mostly using it as a reference when building a toy-ish F# library, but there’s one thing I’ve failed to grasp and that’s the ‘Option’ type.

How does it work and what is it’s real world usage?

Answer 1

The option type is at least similar to Nullable<T> and reference types in C#. A value of type Option<T> is either None which means there’s no encapsuated value, or Some with a particular value of T. This is just like the way a Nullable<int> in C# is either the null value, or has an associated int – and the way a String value in C# is either a null reference, or refers to a String object.

When you use an option value, you generally specify two paths – one for the case where there is an associated value, and one where there isn’t. In other words, this code:

let stringLength (str:Option<string>) =   match str with   | Some(v) -> v.Length   | None -> -1 

is similar to:

int StringLength(string str) {     if (str != null)     {         return str.Length;     }     else     {         return -1;     } } 

I believe the general idea is that forcing you (well, nearly) to handle the ‘no associated value/object’ case makes your code more robust.