[CHY872]

He's making the argument that functional programming is bad. If it were a terrible design decision then the language precluding its implementation (as he claims functional languages do) would be a positive or at least not a negative. So, he must be making the argument that it is a good thing, or at least a necessary thing.

Even so, the ML's all support this type of behaviour (with globals).

In the same way, we do not criticise functional languages for not allowing goto style control flow.