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On 24/07/2012, at 2:50 PM, wrote:

> It looks like setting a key in a hash with value nil is the
> same as if the key does not exist at all.

It is exactly the same.  Setting a table field to nil is the same as deleting the field.

> How can I decide between both in a program?

You can't.  If you need to distinguish between the two, you could try:

null = setmetatable({}, {__tostring=function() return "null" end})
a = { key = null }

But your "null"s will occupy space in the table.