F# Option types Use Option<'T> over null values
Example
In functional programming languages like F# null values are considered
potentially harmful and poor style (non-idiomatic).
Consider this C# code:
string x = SomeFunction ();
int l = x.Length;
x.Length will throw if x is null let's add protection:
string x = SomeFunction ();
int l = x != null ? x.Length : 0;
Or:
string x = SomeFunction () ?? "";
int l = x.Length;
Or:
string x = SomeFunction ();
int l = x?.Length;
In idiomatic F# null values aren't used so our code looks like this:
let x = SomeFunction ()
let l = x.Length
However, sometimes there's a need for representing empty or invalid values. Then
we can use Option<'T>:
let SomeFunction () : string option = ...
SomeFunction either returns Some string value or None. We extract the
string value using pattern matching
let v =
match SomeFunction () with
| Some x -> x.Length
| None -> 0
The reason this code is less fragile than:
string x = SomeFunction ();
int l = x.Length;
Is because we can't call Length on a string option. We need to extract the
string value using pattern matching and by doing so we are guaranteed that the
string value is safe to use.
