[int_19h]

R is lazy, because it does not evaluate arguments upon function call unless and until used. It is also unusual in that the function can avoid evaluating the argument at all, and instead ask for the quoted expression that produced it (which can then be evaluated manually at the desired point, or multiple times, or in a different environment etc), but that is orthogonal to laziness.

To be even more precisely, R itself is lazy "all the way through". Because literally every expression in R is syntactic sugar for a function call (including assignments and control structures such as "if"), the only thing that a function can really do with an argument is pass it on to another function, so, strictly speaking, there's no distinction between use and non-use even. It's just that any R function, in order to do something useful, will ultimately call some non-R leaf function implemented in native code, and some of those leaf functions will actually do the eval if they're defined in terms of argument values (e.g. obviously addition needs to do so to actually compute the value etc).