You can write functional code in any language, just as you've shown. But your mistake perfectly elucidates the point. When you call a function in a functional

You can write functional code in any language, just as you've shown. But your mistake perfectly elucidates the point. When you call a function in a functional language, you know the variable you pass as the parameter won't be mutated. When you call a function in a non-functional language, you only hope it won't. And even when the type system allows you to know the input variable won't be mutated (say, passing by copy in C++), you don't know what other kinds of side-effects the function may be having.