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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?

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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.)

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