[eyelidlessness]

I’m assuming the “free” part is referring to referential transparency. I may have wildly misunderstood that and they may be even more mislead. But when referential transparency is assured, one of the promises of FP is that repeated calls to expensive functions can be optimized. Like I said in my sibling comment, the important part of that promise is can. If a runtime or compiler is able to flag a pure function as expensive, it can automatically memoize it (of course then trading memory space for computation time).