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On 21-Dec-04, at 9:04 PM, Mark Hamburg wrote:

The following is a non-ideal example, but the idea is that if one adopts the
convention that libraries should return errors rather than throwing
exceptions (see, then it
would help to have a way to express errors when false and nil are valid

Surely the pcall convention (first result is the success indicator) does
just that?

function itemExists( db, itemKey )
    -- Returns true if database db contains an item with the
    -- given key. Returns false if the database does not contain
    -- an item with given key. Returns error in the event of an
    -- error reading the database.

Which this example is perfect for:
  true, true (or perhaps some more useful value)
  true, false -- no such key
  false, "Wrong password, bozo" -- database error

Example usage:

exists = error_assert( itemExists( db, itemKey ) )

where error_assert is (in lua 5.1, I think):
  function (success, ...)
    if success then return ...
     else error((...))