Nilpotent

A nilpotent Routine is one that has no observable effect aside from its return value and its consumption of CPU cycles. Calling a nilpotent routine N times (and ignoring its return value) is equivalent to calling it zero times.

Nilpotency is a weaker guarantee than ReferentialTransparency. Consider the following code:

var x = f(3)
return g(x, x)

If f is referentially transparent, x can be inlined without changing the meaning of the program:

return g(f(3), f(3))

However, this is not generally possible if f is nilpotent. f might create and return a mutable Object, and the logic of the program might depend on g receiving the selfsame object as both of its arguments. This might happen if g alters the State of its arguments in any way.

Note that, by the same token, it is not generally safe to extract variables for the result of a nilpotent routine: given return g(f(3), f(3)) we cannot assume that the repeated f(3) call can be extracted to a variable.