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Is there some .NET platform / interop issue that requires F# to have null references? SML and OCaml have Option types and Haskell has the Maybe Monad. It seems weird that F#, being an ML-family language, requires checking for references to null.

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Idiomatic F# code also uses Option types. However, all .NET reference types have null as a possible value. While most F# types don't have null as a "normal" value, there's nothing to prevent null instances of these types from being created "abnormally" at the .NET runtime layer (e.g. values of F# types created in other .NET languages like C# that don't respect F#'s metadata annotations, or created via F# operators like Unchecked.defaultof<_> that implicitly use the .NET runtime's default value of null for reference types).

In practice this is rarely a problem, but the article is highlighting what you need to do when you want to (efficiently) check for these boundary cases.

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