The language, F#, is available free. Either the F# Foundation or part of Visual Studio install.

It is ML based and runs on .NET platform and Mono. So can be used anywhere C# is. This includes phone space with xamerind and the like.

As secure as any other .NET language.

The big difference is NO variables. Everything is const and immutable.

let a = 10
a = a + 1 // Compile error

So requires a very different way of thinking to imperative languages (OO, procedural etc)