Comment by jwarden

Right, so for a subset of functions, a language could implement an extensional equality test operator `==` for two functions by calling the functions for every possible input. It would be prohibitively slow for some functions, but correct.

But for other functions, even that won't be possible.

The point is that functions and arrays may be practically different. You can always do an `==` test on the contents of two arrays, but you can't do the same for two arbitrary functions.