As an Electronic Engineer, my programming experience started with Assembly and continue with PL/M, C, C++, Delphi, Java, C# among others (imperative programming is in my blood).

I'm interested in add to my previous knowledge, skills about functional programming, but all I've seen until now seems very obfuscated and esoteric.

Can you please answer me these questions?

1) What is the mainstream functional programming language today (I don't want to get lost myself studying a plethora of FP languages, just because language X has the feature Y)?

2) What was the first FP language (the Fortran of functional programming if you want)?

3) Finally, when talking about pure vs. non pure FP what are the mainstream languages of each category?

Thank you in advance