I'm working on a big c# app right now. All over the code base are areas where we always assumed a call to .first() would always return at least one thing. The business needs have changed and now we have use cases where we return nothing. I've been playing a game of hunting down the many little areas where this could crop up.

I can imagine that in f# this refactor would be a lot simpler and would allow me to have a case of empty that I could propagate around and handle transparently.

How would functional programming help you in this department?

In C#, null is just one possible value of the regular type Foo. The compiler cannot tell whether you have started returning null where you previously only returned non-null values, nor can it tell whether you've handled those null values correctly at the consumption site.

In F#, None is a subtype of the Option<Foo> union type, which is different from the plain Foo type and from the other subtype Some Foo. If you change a function that used to return a plain Foo into one returning Option<Foo>, every place in your code where that function is consumed will throw a compile error until you edit it to unwrap and handle the distinct Some Foo and None subtypes.

Many functional programming languages have stronger type systems than C#. They typically don't accept null as a valid substitute for any type. To represent a nullable type, you wrap the value in a generic container. Think of it as being like an array that contains at most 1 item.

The interface of this generic container only allows you to interact with the (potentially) wrapped value in a way that is type safe. That doesn't completely eliminate having to think about how the business logic should differ for missing values, but it ensures that you'll never get a null pointer exception.

There is no way (from a type perspective) to guarantee in C# that an instance of a class can never be null -- this has to be ensured by code logic, conventions, etc. As application gets bigger it becomes easy to overlook a case that leads to a null reference exception.

The type system in F# can guarantee that an instance of a class can never be null and you can always refer to it safely.