Re: Dedicated error value/type for Lua?

[Mark Hamburg <mhamburg@...>]

That is certainly the way to deal with this within Lua as it stands, but the
conventions used in FinalizedExceptions and for routines like io.open avoid
having a separate success code by overloading the first result.

Mark

on 12/21/04 9:10 PM, Rici Lake at lua@ricilake.net wrote:

> 
> 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
> 
>