[hadley]

It was my understanding that R basically already does implement call-by-name - arguments to a function are passed by name and looking up in the calling environment until you first modify them. Is my understand incorrect, or do Algol 60's call-by-name semantics mean something different?

[radfordneal]

Sort of. That's why it shouldn't be too hard a modification to implement. For a call-by-name argument, you just have to evaluate the "promise" every time, rather than just the first time. (Assignment to a call-by-name argument will be a bit trickier, but not impossible.)