Re: Dedicated error value/type for Lua?

[Rici Lake <lua@...>]

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 http://lua-users.org/wiki/FinalizedExceptions), then it
> would help to have a way to express errors when false and nil are valid
> results.

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((...))
    end
  end