Fortress 0.1 Alpha, 39 bytes.

f(n)=if n<3then n else f(n-2)-f(n-1)end

This language is old and dead, very dead. (1-indexed: f(1)=1, f(2)=2).

Can be placed into a program like this:

export Executable
f(n)=if n<3then n else f(n-2)-f(n-1)end
run(args) = do
    println(f(3))
end

Does this really need explaining? This is just the definition.