Just because you use some intermediate variables to calculate f(x,y) = x^2 + y^2 doesn't make it a non-pure function. At least at the level of abstraction we're

Just because you use some intermediate variables to calculate f(x,y) = x^2 + y^2 doesn't make it a non-pure function. At least at the level of abstraction we're talking about (the API boundary). The more significant application of storage will be long-term storage wrapped in a read-modify-write loop.