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On Thu, Sep 29, 2016 at 9:33 AM, Yong Wang <> wrote:
here is 41-byte solution:

function NVL(x,y)return(y..x):sub(-#x)end

You've found it!
I believe it was not easy.

This solution is based on strange behavior of string.sub.

IMO, "string.sub(index_from, index_to)" will be more handy if "index_from = 0'
would mean "the index after the last character"
instead of "the index before the first character".
("index_to = 0" should mean "index before the first character", as it is currently implemented in Lua)

The unexpected result of "str:sub(0)" makes programming a bit harder,
as this "feature" is actually a trap, and you are compelled
to use additional "if" to make things right.
But surprisingly, this weird logic can be beneficial in codegolf :-)