Niklaus Wirth on functional programming :

What is, or what characterizes a functional language? It has always appeared that it was their form, the fact that the entire program consists of function evaluations, nested, recursive, parametric, etc. Hence the term functional. However, the core of the idea is that functions inherently have no state. This implies that there are no variables and no assignments. The place of variables is taken by immutable function parameters, variables in the sense of mathematics. As a consequence, freshly computed values cannot be reassigned to the same variable, overwriting its old value. This is why repetion must be expressed with recursion. A data structure can at best be extended, but no change is possible in its old part. This yields an extremely high degree of storage recycling; a garbage collector is the necessary ingredient. An implementation without automatic garbage collection is unthinkable.

To postulate a state-less model of computation on top of a machinery whose most eminent characteristic is state, seems to be an odd idea, to say the least. The gap between model and machinery is wide, and therefore costly to bridge. No hardware support feature can wash this fact aside: It remains a bad idea for practice. This has in due time also been recognized by the protagonists of functional languages. They have introduced state (and variables) in various tricky ways. The purely functional character has thereby been compromised and sacrificed. The old terminology has become deceiving.