Re: error() or nil, 'error msg'

[Jorge <xxopxe@...>]

On 09/19/2014 04:39 PM, Andrew Starks wrote:
> Hisham / Jorge:
>
> How do you deal with the following:
>
> 1: You parse the input that you receive and you know it to be well-formed.
> 2: You call a library function that will look for a resource by that
> name and, if it fails, returns `nil, error`.
> 3: That same call may also generate an error. For example, perhaps the
> object contains a dependent resource that is in an invalid state.
First, as I said, I don't expect to library to crash me. Then I accept 
the library might be unable to return anything back, for _any_ reason.
This means there's a "if ret then" or equivalent somewhere. 
Alternatively, if this result is absolutely critical for me continuing 
doing stuff, I might wrap the call in an assert. I could also do that 
while developping, to reduce the number of cases I have to handle.
If there's no assert, and I got no result, then I might ask what 
happened. In your example, I would expect to find in the docs the 
"resource does not exist" error, and perhaps a bunch of other weird 
error conditions. Probably all bundled in a "unknown error" :)
In other words, I either have a result, or not, and then all the reasons 
why not are in one single place.
So my app would be something like:

local ret, err = lib.call('resource_name')
if ret then
	--dostuff with ret
else --as long as I care
 	if err == "does not exist" then
		--think hard what to do and why i'm here
	else
		error(err)
	end
end

It's a bit like the socket API and the EAGAIN, which is barely an error 
at all, and all the other errors: they are very different conceptually, 
all they share is that there's no data returned, but are all handled in 
the same manner.

Jorge