[willtim]

There is no well defined definition of functional programming. Since almost every mainstream imperative language supports higher-order functions, I would suggest that criteria is inadequate. A better definition is programming without side-effects.

[rwjwjuwjudf]

Output is a side effect.

[willtim]

Not in Haskell and other pure languages. Pure functional programming is used to construct larger programs from smaller ones, which only when finally executed by the runtime produce output.

[rwjwjuwjudf]

Are there any programs that produce output before they are executed?

[willtim]

It is not easy describing pure functional programming in two sentences. I encourage you to read up on it yourself. Again, IO is not a side-effect in pure languages such as Haskell. And again, the absence of side-effects best defines functional programming, not any particular feature, which imperative languages can and do steal.